The rider is the part of the contract between a performer and a promoter that specifies what the performer requires before, during and after a show. This covers such mundane things as crew catering and load in/load out time, but it also involves the specific needs of the artist.
Elton John is currently on tour in Brazil. His rider requests an extra hotel room in which he can store his glasses. And woe be to the person who forgets the chrysanthemums in the dressing or has the room temperature above 15 degrees.