One of the Linux distributions.
Linux is one of the OS (Operating System). In addition to Linux, there are Mac OS X and Windows OS. Linux is an open source operating system that users can use to develop new operating systems. The result is called a distribution. And back to the story, Ubuntu is a distribution developed based on the OS called Linux. (Strictly speaking, Ubuntu was also based on the Linux distribution Debian)
Below, other words of interest (planned to be investigated) Docker Kernel Virtual box Shell Bash yml