A belayer is the partner on the ground (or at a belay station mid-route) who feeds rope through a belay device, takes in slack, and catches the climber if they fall. The role is foundational — without a competent belayer, roped climbing isn't possible.
Belayers do four things at all times: pay out rope as the leader climbs, take in rope as the climber moves up on top rope, manage slack so a fall results in a clean short catch, and lock off the device instantly the moment the climber falls. They also watch the climber, communicate with them ("clipping," "take," "slack"), and deliver "soft catches" by stepping or jumping to absorb the fall force gradually rather than bouncing the climber off the rock.
Good belaying is a learned skill. Indoor gyms test belayers before issuing a lead-belay licence. Common mistakes include too much slack (long fall), too little slack (rope drag), short-roping (sudden hard stop), and mid-fall hand position (gripping the brake strand wrong, slipping rope through the device). A trusted belayer is one of the most valuable parts of a climber's circle.