Docker is a Container technology. Think of containers as even more stripped down VMs, that only exist for a single purpose. Instead of having a VM template that has all of the preconditions you need for testing that you create on demand and remove when done, containers would have an individual container per application or service you needed which would be spun up on demand, and automatically destroy itself once used.
The advantage is that they’re even more lightweight, interchangeable, and have even fewer dependencies than VMs. They’re designed for single tasks, so they have disadvantages for any kind of hands-on testing, but for automation and pipelines they’re amazing.