Main cast refers to actors and actresses who received cast billing in the opening credit sequence of a television series. These are the main stars of the series and are credited in each episode per season that features them, though often the cast member may not even appear in the episode. This is a pretty sweet deal for some, as they get paid for working an ep even if they aren't in it. Some supporting cast members may eventually become main cast members as a series progresses depending upon contract negotiations and the prominence of their respective characters.
Example: On the AMC series The Walking Dead, actor Norman Reedus had a guest starring role in season one, and his name was listed after the main cast and production crew credits during the first act of each episode. The character quickly became a fan favorite, and by season two, he was elevated to a starring role on the series.