Time trouble refers to the critical situation where a player is running dangerously low on the clock, to the point where time management becomes as decisive as the quality of the moves played.
The term is borrowed from German: "Zeit" means "time" and "Not" means "distress." A player in time trouble typically has less than a minute left to complete several moves — sometimes ten or more. Under these conditions, deep calculation becomes impossible: moves are played on instinct, and serious blunders multiply. A winning position can collapse into a draw, or even a loss, within seconds of careless play.
In practice, the best way to avoid time trouble is to build good clock discipline from the very first move: check your clock regularly, avoid spending too long on a single position, and accept a reasonable move rather than searching endlessly for the perfect one. If your opponent is in time trouble, keep the position complex and resist the urge to simplify — the clock pressure will do the work for you.