Docker is a container technology developed for Linux. However, if you are new to the world of software development and/or server computing it is difficult to wrap your head around what purpose does Docker actually server.
A good starting point to consider would be hypervisors like VirtualBox or VMWare which most of us have used at some point to run a different operating system (called the guest OS) on top of the current operating system (the host OS a.k.a hypervisor) that we are currently running.