What does co-star mean?

Answer 1

It usually means two or more actors of the same career stature appearing in a project together. An example would be "Boeing Boeing" (1965). Tony Curtis and Jerry Lewis were of roughly equal stature at the time and top billing was a major contractual issue in getting them to appear. It was finally agreed that in the opening credits their names would rotate like an airplane propeller. That way neither name would appear first.