Hi Rachel,
Really the big question is why they don't line up more often. The picture below is from http://www.mreclipse.com/Special/SEprimer.html



It shows the sun, earth, and moon from above (not to scale!). When the moon gets directly in between the sun and earth (in other words when the sun, moon, and earth line up) some people on the earth can't see the sun and we have a solar eclipse.
The moon goes around the earth almost once a month. From that you might think the moon should come between the earth and sun once a month, but it does not.
That is because the moon and earth do not orbit in a flat plane like a piece of paper. The moon actually moves above and below the line between the Earth and Sun and only sometimes comes right between them.
this is show in this picture from
http://starryskies.com/Th...r-2003/eclipse2.html



Hope that helps explain it.
Terry