Container
A standard package holding a program and everything it needs to run, so it behaves the same on any machine.
What it tells you about a developer
Container experience is table stakes for backend and infrastructure roles, so treat its presence as confirmation. The useful question is what someone did with containers. Packaging an app for deployment is everyday work. Designing how dozens of containers run, talk to each other, and recover from failure points to platform and infrastructure depth.
A container bundles a program with its dependencies: the runtime, the libraries, the settings. The bundle runs identically on a developer's laptop, a test machine, and a cloud server, which removes a whole class of 'works on my machine' problems. Docker made containers mainstream, and Kubernetes manages them at scale. Most cloud deployments today ship as containers.