Docker is (from their site) “an open platform for developers and sysadmins to build, ship, and run distributed applications.”

We deploy components of streaming system into Docker containers to make use of the isolation and ease of deployment that Docker containers provide.

You can think of a Docker container much like a stripped virtual machine, except typically they are running just a single application or service and they lack the resource overhead that virtual machines typically consume. For more on Docker and the Docker philosophy, check here.