A transit has a specific meaning in the field of astronomy. It is when a planet, asteroid or moon passes in front of a star. A moon passing in front of a planet is also a transit. To our eyes, a transit usually looks as though a small, dark dot is crossing the face of a star or planet.
Transits are useful to astronomers searching for new planets outside of our solar system. If the scientists see the light from a star dip a little, this could be because a planet is in transit in front of that star.