Load balancer
A service that sits in front of several servers and spreads incoming traffic among them, so no single server gets overwhelmed.
What it tells you about a developer
Load balancer experience means the system had enough traffic to need more than one server, which is itself a scale marker. The load balancer sits at the front door of a system, so mistakes there are visible to every user at once. A developer who has configured and operated one has worked at that pressure point and debugged live traffic problems.
Once a system runs on more than one server, something has to decide which server answers each request. That is the load balancer. It also checks server health and routes around machines that stop responding, which makes it a core piece of high availability. Nginx and HAProxy are common tools, and every cloud provider sells one as a managed service.