A main character, commonly abbreviated to MC, is a character that takes up the most important role in a story or (role-playing) storyline. It is almost always the same person as the protagonist. The antonym of the term main character is side character (SC).
Novels and other media that tell stories often have one or a couple of main characters. Single-player role-playing games do as well, but most MMORPG's and textual role-playing games make no such distinction. The only important distinction there will be between playable and non-playable characters.