Definition of stop loss
A stop loss is a loss limit that a player sets for himself. This limit can be set in cash games, tournaments and sit'n'gos. Variance is part of the game. It's normal to win and lose buy-ins, but beyond a certain limit in a certain amount of time, say 10 full cash games in a few hours, it may be time to take a break, at least for the evening. There are online software programs to help us. After X number of losses, either in cash games or tournaments, the software will shut down and block all our tables for us.
Variance, tilt or fatigue?
Imposing a stop loss can be a good thing if you know yourself well. Few players can continue to play their A game after losing a lot of buy-ins. If you are one of the few players who can continue to play your A game after so many losses and still be objective enough to determine that your losses were due to variance, you may not need such a limit, but for the majority of players, the stop loss can be a practice that avoids disaster to the bankroll.