"It means "trim condition" i.e. the airplane is flying well inside its envelope and is not subject to large pitching and yawing moments. The autopilot will attempt to trim up until it is overwhelmed or manually disconnected. In that video, we can see that the trim condition suddenly maxes out as he begins his turn at too low an air speed, and as soon as the autopilot disconnects, the upset begins. (Correct me if wrong, I am not a professional pilot.)
-drl"
Completely wrong, I'm afraid. See above.