The Docker (https://www.docker.com/) project is a container platform, which lets you run your applications in isolated environments. Docker leverages existing Linux technologies like cgroups (https://en.wikipedia.org/wiki/Cgroups) to provide a set of high-level tools to drive a collection of running processes. On Windows and macOS, Docker interacts with a Linux Virtual Machine, since a Linux kernel is required.
As a Docker user, you just need to point which image you want to run, and Docker does all the heavy lifting by interacting with the Linux kernel. An image in that context is the sum of all the instructions required to create a set of running processes on the top of a Linux kernel to run one container. An image includes all the resources necessary to run a Linux distribution. For instance, you can run whatever version of Ubuntu you want in a Docker container even if the host OS is a different distribution.