Big Bad is a term originally used for the television series Buffy the Vampire Slayer to describe its major antagonists but has since been used by some critics and fans for multiple other shows, including Supernatural.
The title is divided into the main villains and the secondary villains. The Big Bad is the main villain of the season, orchestrating events behind the scenes, while the secondary villain(s) often (but not always) serve the Big Bad, appearing more regularly and antagonizing the heroes. In some cases, there are several antagonists that also work against each other, other than just antagonizing the Winchesters. Some Big Bads are introduced early on into the season and act as the Big Bad throughout the major arc, while some are later revealed to simply be secondary villains, and are then replaced by the season's true Big Bad or some other character also playing an antagonist role.