When a pianist speaks about his or her "repertoire", it means all the pieces that he can play. Most famous musicians have a wide repertoire (they can play a lot of pieces).
The "piano repertoire" means all music written for the piano. The violin has a much larger solo repertoire than the viola. That means that much more solo music has been written for the violin than for the viola.
A theatre company may have a "repertoire". That means all the plays that they regularly perform. Sometimes theatre companies are called "repertory theatres" because they have a particular list of plays that they perform.
Repertoire Facts for Kids. Kiddle Encyclopedia.