Linux Distros short for Distributions are just different versions of Linux created by other people. Since Linux is an open-source operating system, anyone can make their own version of it and call it whatever they want. Ubuntu for example is a Linux distribution. It uses the same code, but expands upon it to make it their own. Ubuntu has other versions like Kubuntu, Edubuntu, Xubuntu and a few more. Kubuntu is like Ubuntu but uses a different file system I believe. Xubuntu I think is supposed to be a low resource OS that can run on older hardware and Edubuntu is more geared towards education purposes.
There are a ton of different ones out there, Wikipedia has a good list of them, even puts them in their own categories - https://en.wikipedia.org/wiki/List_of_Linux_distributions