Complex sentence: If a planet crosses (transits) in front of its parent star's disk, then the observed visual brightness of the star drops by a small amount; depending on the relative sizes of the star and the planet.
Split into simpler sentences: If a planet crosses (transits) in front of its parent star's disk, then the observed visual brightness of the star drops a small amount. The amount the star dims depends on the relative sizes of the star and the planet.