Kubernetes
Also known as: k8s
Kubernetes is an orchestrator: it runs containers across a group of machines, restarts them when they fail, moves them when a machine dies, scales them with demand, and rolls out new versions gradually.
It is genuinely excellent at that, and it is a substantial system in its own right — networking, storage, access control, upgrades of the cluster itself. Adopting it means someone now operates Kubernetes in addition to operating your application.
It is worth it for many services, several teams and real scaling requirements. Below that, a couple of servers running containers behind a load balancer does the same job with a fraction of the moving parts, and can be understood in an afternoon by whoever inherits it.