An error made on a shot the player was expected to make, without being forced into a difficult position by the opponent. Hitting a routine forehand into the net when you had time and balance? Unforced error. Missing a running backhand at full stretch? That's a forced error. The distinction is subjective and stats companies don't always agree, but unforced errors are the stat that tells you when a player is beating themselves.
"Wide open court, all the time in the world, and she puts the forehand long. That's an unforced error. She knows it too."