During a solar eclipse, the Moon and the Sun appear to be of similar size from the Earth. This is called apparent size*. It depends on both the object's actual physical size and its distance from the observer.
Even though the Moon's actual physical size is much smaller than the Sun, the Moon is much closer to the Earth than the Sun. Because of the difference in distances, the apparent sizes of the Sun and the Moon are nearly the same when viewed from Earth. When the Moon passes directly between the Earth and Sun, the Moon's apparent size is large enough to block the Sun's light, resulting in a solar eclipse.