If a planet’s orbit around its star is angled just right, the planet will regularly pass in front of or “transit” its star from our point of view. When it does, the planet will block a tiny bit of the star’s light.
Astronomers can search for planets by watching for slight, periodic changes in the brightness of a star. If the brightness of a star changes regularly and consistently, it could mean that a planet is repeatedly transiting the star and blocking a little bit of starlight each time around.