Every month, the motions of the moon in relation to the earth and the sun produce the phases of the moon, described here and here with diagrams.  

When the moon is positioned on the far side of the earth with respect to the sun (so that the side receiving the full force of the sun's rays is towards observers on earth) we see a full moon, and when the moon is positioned between the earth and the sun (so that the side receiving the sun's rays is towards the sun and invisible to observers on earth) we have a new moon (this post from a new moon earlier this year also contains a video of the mechanics involved).

If the moon's orbit were on the identical plane that the earth follows around the sun (the plane of the ecliptic), then every time it passed through the exact point of new moon and full moon, observers on earth would see an eclipse.  There would be a lunar eclipse on every full moon as the shadow of the earth crossed over the face of the moon, because a full moon takes place when the earth is between the moon and the sun.  There would be a solar eclipse on every new moon as the interfering body of the moon inserted itself between the earth and the sun, because a new moon takes place when the moon passes between the earth and the sun.  

However, because the moon's orbital plane is inclined to the plane of the ecliptic, the moon's orbit only passes through the ecliptic plane twice -- once "on the way up" and once "on the way down."  These are known as the lunar nodes, one known as the "ascending node" and the other as the "descending node."  These nodes and their motion are described in much greater detail in this previous post, which also discusses the likely reason for "seven-world cosmologies" and "nine-world cosmologies" and how the nodes fit in (the seven worlds represent the five visible planets plus the sun and the moon for a total of seven, while the nine worlds include the two lunar nodes as well for a total of nine).  For further discussion, see also this post, which contains a link to excellent animation on the subject.

When the moon passes through the point of new moon or the point of full moon at the same time that it is passing through one of the two lunar nodes, observers on earth can see a solar eclipse or a lunar eclipse, respectively.

This month, new moon will take place on May 20 (it will already be early on May 21 in Greenwich time and the "leading" time zones of the globe), and this new moon will be special because it will create a solar eclipse.  Observers in the portions of the earth that can observe the sun will be able to observe the eclipse.  Because their location on the earth will determine the angle between themselves and the two heavenly bodies of moon and sun, the eclipse will be partial in most latitudes (the moon taking a bite out of the sun of varying sizes depending on the observer's latitude), but within a certain band of latitudes (which will move like a rifle barrel along the turning earth during the day) the moon and sun will align to create an annular solar eclipse.

The distinction between an annular solar eclipse and a total solar eclipse results from the apparent size of the moon, which changes throughout its elliptical orbit (recall that we were recently treated to a full moon of unusual apparent size).  The diagram below shows the variations between the maximum apparent size of the moon and the minimum apparent size of the moon (black lines -- the maximum is the outermost line and is solid and the minimum is the innermost line and is dotted) as well as the sun (red lines -- the max is the outermost again, and you can see that the moon's apparent max and min vary much more widely).

If the apparent size of the moon happens to be equal or larger than that of the sun during a solar eclipse along the "rifle barrel" of perfect alignment to an observer on earth, then observers in that band of latitudes will be treated to a total eclipse of the sun, perhaps the most awesome natural celestial phenomenon there is for earthly observers (one will occur in November of this year for observers in Australia and the South Pacific).  

However, if the moon is too far away to completely cover the sun, observers along the narrow band of perfect alignment are treated to an "annular eclipse," named for the Latin word anulus or "ring."  This month's solar eclipse will be an annular eclipse.

This webpage from NASA contains an embedded Google map which depicts the track of the upcoming May 20/21 annular solar eclipse.  You can move the green cursor along the track to see the exact times that the eclipse will begin, reach maximum, and end at any point along the path.  You can also zoom in and zoom out.

Note that looking directly at the sun is extremely dangerous and can cause permanent damage to your retina and even permanent blindness, and even more so if its beams are enhanced by additional optical lenses, and that extra caution must be exercised during the observation of an eclipse for safety (during a total eclipse the effects can be safely observed with the naked eye during totality, but this does not hold true for an annular solar eclipse or for a partial solar eclipse).  This webpage from Sky & Telescope gives advice on safely observing a solar eclipse.  This article also describes the upcoming annular eclipse and gives some additional safety tips.

In her excellent book The Death of Gods in Ancient Egypt, Jane B. Sellers argues that solar eclipses (and total eclipses in particular) are directly related to the ancient myths of the "contendings of Seth and Horus" and the death and resurrection of the gods.

Her explanation is quite thorough and extensive, and should be read in its entirety in her book, but a few quotations she cites from the ancient Egyptian texts should suffice to give an idea of her argument.  For instance, on page 43 of her book, she cites this passage from the Egyptian Book of the Dead:
On the crest of that mountain is a snake, He Whose Consuming Fire is Within Him is his name.  Then after mid-day he will turn his eyes against Re, then a stoppage will take place in the bark and great amazement among the sailors . . . Then Suty will hurl a spear of metal against him and cause him to disgorge all that he has swallowed.
Elsewhere, on page 32, she cites a different passage from the Book of the Dead:

I filled for thee the Eye after it had failed that day of the battle of the Two Fighters.

What then is it?

The day it is of the fighting of Horus with Seth, throwing excrement in the face of Horus . . .
Several previous posts have discussed other aspects of the important theories of Jane Sellers, most notably this one

If you are in a location where it is possible to do so, make your plans to (safely) observe the upcoming annular solar eclipse!