Gitify is a command line program you can use to get data in the MODX database, into files you can commit into a version control system like git. It's meant to be used for dev/stage/production workflows ("acceptance workflows" some people call them), as MODX does not natively allow you to version database content.
Git isn't required, but for the workflows it was meant for, it definitely makes sense to use a version control system like Git, or SVN.
Gitify's two main commands you can run are `Gitify extract`, which will take information from the MODX database, and write that information to a simple file, formatted in YAML. Those files are then committed to your version control system, pushed to another environment (/MODX installation), and with the second command `Gitify build`, it will read the information from the files, and push that into the MODX database.
It also has other utilities for installing and upgrading MODX or third party packages, as well as a quick database backup and restore.
There's also a MODX Extra called Gitify Watch (available for free from modmore.com) which will automatically run the `Gitify extract` command after you've edited a resource or element in MODX, as well as the necessary git commands to commit and push the changed files. That's rather useful for a staging or production environment where you're making quick changes, which also need to be pushed back into your repository.
The official documentation for Gitify is here:
https://docs.modmore.com/en/Open_Source/Gitify/index.html