Before starting, Docker is a very interesting, even revolutionary technology. And is so powerful that has many use-cases and ways to explain what it does.
Here, we are talking about Docker as a tool for packaging and executing our applications, whatever they are.
When Docker was created, many people sold it as 'lighter Virtual Machines' and, while that is true, Docker appeal lays in its ability not to just RUN but to PACKAGE and DEPLOY applications in an isolated and repeatable way.