A load balancer sits between clients and your servers. Clients connect to the load balancer's IP address. The load balancer forwards each request to one of several backend servers.
From the client's perspective, there's one server. Behind the scenes, dozens of servers might handle the actual work.
The load balancer also monitors server health. If a backend server stops responding, the load balancer removes it from rotation. Healthy servers absorb the extra load until the failed server recovers.