What exactly is a solar eclipse?

Every month the solid moon travels an orbit around the earth. At one point of  the orbit the moon passes between the earth and the sun. If it passes in exactly the right place, the solid moon blots out the radiant face of the sun and causes an eclipse.

This does not happen every month because the moon's orbit is a little irregular.  As a rule, it passes above or below the sun. To cause an eclipse, the moon must also be close enough to us to cast its shadow upon the earth. As a rule, it is a little too far away to reach us with its pointing shadow.