We know that the closer an object is to our eye, the bigger is its apparent size i.e the size an object at a distance appears to be.
When the Moon passes directly between the Sun and Earth, and all three are in a straight line, the Moon eclipses (passes in front of and blocks the light of) the Sun. In this position, observers on Earth see a solar eclipse. The Sun is about 400 times larger than the Moon, but the Moon is about 400 times closer to the Earth. The result is that from Earth, they appear to be the same size. And when its orbit around Earth takes the Moon directly between Earth and the Sun, the Moon blocks our view of the Sun in what we call a solar eclipse. In a solar eclipse in which the Moon appears as large as the Sun, the Moon completely covers the Sun. This event is called a total solar eclipse.