The Sun hides behind the Moon from our view when the Moon passes between us on Earth and the Sun, hiding part or all of the Sun from our view. Even though the Moon is much smaller than the Sun, because it is just the right distance away from Earth, the Moon can fully blocks the Sun's light from Earth's perspective.
Many millions of years in the past, the Moon was too close to the Earth to precisely occlude the Sun as it does during eclipses today; and many millions of years in the future, it may be too far away to do so.