As the name implies, a Self-Insert Fic is one where the author has made a simulacrum of him- or herself -- commonly called an avatar -- in the story's world as a key character. If the author has any sense of subtlety, the resulting character won't share their name or alias, but it's still easy to tell who that "new character" is. Played for Laughs this can become almost a different story entirely, which usually involves an average loser realising that whatever fictional world may not be the best or safest place for them, and commenting on the implausible things going on.