Complex sentence: The ceremonial first pitch is longstanding ritual of American baseball in which a guest of honor, usually either a celebrity or a locally significant person, throws a ball to mark the end of pregame festivities and the start of the game.
Split into simpler sentences: The ceremonial first pitch is longstanding ritual of American baseball. Usually thrown by either a celebrity or a locally significant person, the ceremonial first pitch denotes the end of pregame festivities and the start of the game.