High availability
Designing a system to keep working when parts of it fail, usually by running duplicates that take over automatically.
What it tells you about a developer
High availability work means someone has designed for failure on purpose. They assume machines break and think in terms of what happens next, which is the mindset that comes from running serious production systems. A failover they designed that held up when a real outage tested it is strong evidence of that mindset in practice.
Machines fail constantly at scale, so high availability treats failure as normal and plans for it. The core technique is redundancy: two or more copies of every important part, with automatic failover that shifts traffic to a healthy copy when one dies. Load balancers, duplicate databases, and multi-region deployments are the standard building blocks.