A bankable star is an actor

Actor

An actor is a person who acts in a dramatic production and who works in film, television, theatre, or radio in that capacity...

 famous or charismatic enough to be "capable of guaranteeing box-office success simply by showing up in a movie". A bankable director is a similar notion.
Media consultant Blake Harris states that a bankable star is one of the "3 minimum ingredients to any film package", the other two being a successful director and a script or a strong story idea. The involvement of a bankable star in a film gives investors confidence that they will achieve a return on their investments, by ensuring that the film is widely distributed and that at least some people will pay to see it (i.e.