Git is one very popular tool to help you work with code.
In particular, it is part of the version management tools. You might know the names Subversion, Mercurial - those are all equivalent to Git, with some differences.
The open source world in particular has adopted Git in the last decade for various reasons, and it’s quite an essential tool to know - at least the basics.
GitHub is a fantastic website where people freely share the code they wrote, with the hope that other people find useful and can contribute to that.
It’s all based on Git, and its success has greatly influenced the success of Git, as well.
I will point you to two tutorials I wrote about those topics: