The Ruby Version Manager is a command line tool to simply install and manage different versions of Ruby.
rvm istall 2.3.1 for example installs Ruby version 2.3.1 on your machine.
rvm list you can see which versions are installed and which is actually set for use.
user@dev:~$ rvm list rvm rubies =* ruby-2.3.1 [ x86_64 ] # => - current # =* - current && default # * - default
rvm use 2.3.0 you can change between installed versions.