Horizontal scaling
Handling more traffic by adding more machines that share the work, rather than upgrading to one bigger machine.
What it tells you about a developer
Horizontal scaling experience signals distributed systems thinking. This developer has dealt with many machines acting as one: keeping them consistent, surviving individual failures, and finding bugs that only appear when work happens in parallel. That class of problem builds the depth that large-scale backend and infrastructure roles pay for.
Horizontal scaling spreads load across many ordinary servers, with a load balancer dividing the traffic among them. It has no ceiling: when traffic doubles, add machines. The cost is complexity, because many machines acting as one must coordinate, share state, and tolerate any single one dying. Nearly every large internet service scales horizontally.