The moon completes an orbit every 27 days.
The Earth completes an orbit around the Sun every 365.2422 days (it may help to pretend the Sun orbits the Earth)

A solar eclipse happens when a line from the Earth to the Sun intersects with the Moon.

We can figure out when that happens using some circle geometry (we can get better predictions using ellipses):

Pull out a piece of paper and draw two circles, one inside the other and each having the same center. The outer one represents the Sun's fictional orbit around the Earth, and the inner one represents the Moon's actual orbit around the Earth. Draw 3 dots, one at the center of the circles, such that the three dots form a line. The outermost dot is the Sun, the next outermost the Moon, and the innermost the Earth. This is a total solar eclipse.

The Sun "moves" 2*pi radians over the course of 365.2422 days, while the Earth moves that many radians over the course of 27 days.