You may hear this shouted by the climber to the belayer. It’s a request that the belayer “lets out” a small amount of rope, which allows the climber a little more room to move. When starting out it is reassuring to have the slack taken in whilst climbing, however as you progress it is sometimes useful to have a little amount of slack in the rope to allow freedom of movement and balance. Obviously too much slack is dangerous and increases the distance of a fall, should this occur.