The movies has an elaborate system of "billing," the order and presentation of actors' names in credits and on posters and there are probably actual rules, which I don't know. A "starring role" is more important than a "featured role." I am guessing here, but I think that a movie has only one or two "stars" and that other names that are important enough to be shown on posters, etc. are "featured" roles. For example:
I believe the names listed above the title--Humphrey Bogart, Ingrid Bergman, Paul Henreid--are stars, and the names listed below--"with Claude Rains, Conrad Veidt, Sydney Greenstreet, and Peter Lorre" are featured roles.
The "featured" roles are very important. They are seen over and over throughout the movie, and if they weren't there, the stars would have nobody to talk to. But they are not as important as the stars.