Git config allows you to customize how git works. It is commonly used to set your name and email or favorite editor or how merges should be done.
To see the current configuration.
$ git config --list
...
core.editor=vim
credential.helper=osxkeychain
...
To edit the config:
$ git config <key> <value>
$ git config core.ignorecase true
If you intend the change to be true for all your repositories, use --global
$ git config --global user.name "Your Name"
$ git config --global user.email "Your Email"
$ git config --global core.editor vi
You can list again to see your changes.