The term "B-movie" was used to distinguish a film from an "A=movie", in a time before TV when movies were all there were. Movies were cranked out continually, the way TV shows are now. An A-movie is a high-budget, highly promoted feature film. A B-movie is the regular, lower budget stuff.

A movie known for their low budget. Or possibly even their low grade of acting. Most of the time b-movies are either scifi, or horror. But are most of the time unintentionally funny.