Trailing stop
A stop that moves with price in your favour by a set distance, but never back.
In plain English
It locks in part of an open gain as the trade moves, and it can also take you out on ordinary noise if the distance is set tighter than the market normally swings.
Example
A trailing stop 5 below the high: price runs from 100 to 120, the stop rises to 115, then price drops and you exit near 115.