How does a solar eclipse happen?

Solar eclipses are an accident of nature. They are so spectacular   because the Moon and the Sun appear almost the same size. In reality the Sun is much further away then the Moon, but much larger.

The Moon orbits the Earth once a month, and eclipses happen if it lines up exactly with the Earth and the Sun.  Solar eclipses occur at New Moon, when the Moon is between Earth and the Sun. Lunar eclipses occur at Full Moon, when Earth is between the Sun and the Moon. Eclipses do not take place every month because the orbits of the Moon and Earth are tilted at an angle. Most of the time, the line- up is not precise enough for an eclipse. However, there are more eclipses than people are generally aware of:

There are at least two eclipses of the Sun each year, though most are partial.
There are at least two eclipses of the Moon each year, though a proportion of these are only penumbral, when the Moon is not seen to darken by very much.
There can be as many as seven eclipses (solar plus lunar) in any one year. In 1935 there were five solar eclipses - four partial and one annular.

On average, there is a total solar eclipse visible somewhere about every 18 months. However, from any one location on Earth, total eclipses take place on average only once in several hundred years.