A container is a software package that holds an application with everything it needs to run (libraries, dependencies, configuration), and that runs in isolation on any machine equipped with a container engine. It solves the classic "it works on my machine, not in production".
Unlike a virtual machine, which ships a complete operating system, a container shares the host system's kernel. It is therefore much lighter and starts in seconds, which makes it possible to run dozens of them on the same server.
Docker popularized the format from 2013 onwards; the standard is now open (OCI). When managing hundreds of containers, teams use an orchestrator such as Kubernetes, which spreads them across servers, restarts them on failure and adjusts their number to the load. Containers have become the standard way to deploy applications in the cloud.
Why it matters when hiring
Using containers is now expected of most back-end developers; operating them at scale is a job of its own (DevOps, SRE, Platform Engineer). In an interview, tell apart someone who writes a Dockerfile from someone who can secure images, troubleshoot a container stuck in a restart loop or size a Kubernetes cluster.