Have you ever tried to install Minecraft and seen an error message like, “This application requires a Java Runtime Environment 1.6.0”? Or you try to install something on Windows, and you get an error that says some .NET framework is missing? Or, as a more basic example, have you ever spent a couple hours setting up a new computer with all your applications and preferences?
Those are the kinds of problems Docker, and “containers” more broadly (Docker is kind of the Kleenex of containers), are meant to solve. Docker makes it easy to install Linux applications on servers, along with their required dependencies and whatever preferences you might have for those applications. And, as an added bonus, conflicting dependencies between applications (maybe one app relies on Python 2, and another app relies on Python 3) aren’t an issue, because everything is isolated in different containers.