Trailing stop
A trailing stop is a stop loss that automatically follows your position as price moves in your favour, at a set distance.
If you buy EUR/USD with a 30-pip trailing stop, your stop moves up 1 pip every time price makes a new high 1 pip above the previous one. If price falls back, the stop stays put. It never moves backwards.
It lets you ride a trend while locking in part of the profit. Its weakness: too short a distance takes you out on a simple pullback, too wide a distance gives back much of the gain.
On MetaTrader, the trailing stop runs on the platform side: it stops trailing if your terminal is closed. Some traders prefer to move it by hand below each new swing low, or based on the ATR.