A transit is an event in which a planet passes in front of its parent star, temporarily decreasing the star’s brightness by a small amount. Imagine looking at a small planet orbiting a star far, far away. Occasionally, the planet might pass in between you and its star, briefly blocking some of the starlight. If this dimming happened with enough frequency, you might be able to infer the presence of the planet, even if you can't see it. This, in essence, is the transit method of detecting exoplanets.