The easiest comparison for Docker would be that it acts somehow as kind of virtual machine. The differences are, that you don't simulate a whole system, but just a part of the existing system. That makes it also much more performant than a VM.
Instead of installing the application and its requirements on the server itself, it's all installed in the docker container. So you create a docker container for an application like avorion server and run it on any system docker supports (with some restrictions). So an application can be defined / configured once and is then portable.