Ruby Version Manager (RVM) is a tool that allows you to install and update multiple versions of Ruby and Rails on your computer. Some projects were built using older versions of these languages, and aren't compatible with new versions, which is why a version manager is so important.
RVM isn't going to take a lot of your time or brainspace, but here are a few tutorials which give you background on the software and the commands:
Installing RVM - RVM
Step by step, line by line installation instructions for all operating systems.
A Beginner's Look at RVM - Neanderslob
This is a short and simple explanation of how to install and set up RVM and multiple Ruby versions.
How To Use RVM to Manage Ruby Installations and Environments on a VPS - Justin Ellingwood
This is a more advanced tutorial on how to manage multiple Ruby versions, catering to Linux OSs, but could be modified to work with other OSs.