In a nutshell

A cgroup (short for control group) is a Linux kernel feature that lets you manage how many resources (e.g., CPU time, memory, network bandwidth, I/O) can be consumed by a group of processes.

Docker uses cgroups to provide resource isolation and management for containers. For example, you can configure a Docker container so that it never uses more than 50% of the CPU or more than 1GB of the RAM.