What we call planetary "transit" is the passage of a planet in front of the disk of a star. This phenomenon occurs when the star, the planet and the observer are in alignment with each other.
This phenomenon is observed in the solar system - Mercury and Venus recently passed in front of the Sun.
The occultation of a part of a stellar disk by the planet results in a temporary decrease of the brightness of the star. To detect planets, we therefore try to measure this faint brightness variation.
The transit method is now the only technique capable of detecting terrestrial planets.
This is the method used by CoRoT, as shown in this simulation : Transit simulator.