Low-budget motion pictures made usually to accompany major features on double-bills. In the 1930s, "B" was the designation given to a motion picture shown in a U.S. theater as the second half of a double feature. Typically they have lower production values and less well-known or unknown actors compared to major features. The term is still used although the practice of showing two features for a single admission price no longer applies to new releases.

Source: Art & Architecture thesaurus

http://vocab.getty.edu/page/aat/300263871