.idea is the dir for saving the project configurations for all Jetbrains IDES (RubyMine , Pycharm , PHPStorm , WebStorm ..etc)
you can handle it using two ways if you don't want to commit it to the repo
Ignore it only for yourself
in .git/info/exclude
add /.idea
Ignore it in .gitignore so it will be ignored for everyone who uses the repo
by adding /.idea
to .gitignore
if the dir .idea already tracked by git you will need to remove it first from the cached files before ignoring by git rm -r --cached .idea
This folder can include important configuration if you did any custom configuration for the project and also include the indexed data for the IDE which helps it to provide quick autocomplete and in certain cases would be better to commit it to the repo but I always ignore it because the other developers in the team don't use RubyMinee