The Moon is able to eclipse the Sun because it’s distance from the Earth is 400 times closer than the Sun and is about 400 times smaller. So they look similar in size.

But there is a twist..

The Moon can sometimes cause an annular eclipse where the Sun can be seen behind it. That’s because the Moons orbit is elliptical and has two focuses. So in the furthest position, it’s apogee, means that it appears slightly smaller. Hence an annular eclipse. The Moon is gradually receding from the Earth. The mirrors left on the Moon by apollo astronauts are used to check it’s distance. By using lasers reflected from them they have found it is moving away by about 3.8cm a year. Eventually it will be far enough away that no full eclipses will happen anymore.