A percentage of the future net profits of a movie, accepted as payment by an actor or writer. In this case, 'monkey' is intended to be derogatory.
This term was coined by Eddie Murphy, who also stated that only a fool would accept net points in their contracts; always insist on gross points. This is due to the infamous Hollywood accounting, where a studio manages to list every conceivable expense associated with running a studio as an expense of the film in question -- eating up any gross profits.
Writers and other lesser persons often get stuck with monkey points. Being a gross player, someone with enough clout to be given gross points, is not common in Hollywood.