What is Docker and why use it?

An image is a filesystem plus the configuration needed to start a process; a container is that image running as an isolated process on the host kernel, with its own view of the filesystem and network. It is not a virtual machine — there is no second operating system booting — which is why containers start in a moment and are cheap enough to run several of on a laptop.

Containers are deliberately disposable, so anything that must survive a restart has to live outside them in a mounted volume or a managed database. The other thing worth learning early is layer caching: ordering a build so dependencies are installed before source is copied in turns a slow rebuild into a fast one. And Docker alone doesn't run anything in production — that is orchestration, and a separate decision.