When the Moon comes in between the Sun and the Earth in a straight line, then a solar eclipse is formed. This formation of a solar eclipse is based on the concept of:
Formation of a shadow of the Moon on the Earth.
A solar eclipse occurs when the Moon gets between the Earth and the Sun, and the Moon casts a shadow over the Earth. A solar eclipse can only take place at the phase of the new moon when the Moon passes directly between the Sun and the Earth and its shadows fall upon the Earth's surface.