A trailing stop follows a winning trade. For a long position it rises as price makes new highs and stays put when price falls. Once price retraces the trailing distance, it fires like a normal stop-order.
The distance is the key decision. Too tight and normal noise stops you out; too wide and you give back most of the gain. Many traders base it on atr.
Example: long at $100 with a $5 trailing stop. Price rises to $120, so the stop is at $115. Price then falls to $115 and you exit with a $15 gain instead of riding it back to $100.
Related: stop-order, stop-loss, atr, take-profit