Starting pitchers are on a rotation, which is fairly similar to a lineup, but with some important adjustments. Just like a lineup, a rotation dictates the order of starting pitchers. However, unlike a lineup, the pitchers do not all take their turn in the same game. Rather, the rotation assigns one pitcher to each game.
It consists of five (5) starting pitchers; therefore, each pitcher pitches once every five games in the order that the rotation dictates. As long as no one gets hurt, traded, or suspended, the rotation remains the same for the entire season, for the most part. The reason that starting pitchers only pitch once in every five (5) games is because pitching is a very tiring and demanding act, and the rotation ensures that each pitcher has enough time to rest between starts.