Git is a source control system that enables you to keep track of your source code, its history and any changes you make. Git can be used to track any file but is most efficient and best suited when used only with textual files. Because Git is a distributed source control system it works very well to enable groups of people to work on the same source code as well as supporting experimenting with your code, trying out lots of different ideas in separate branches (which are a bit like a copy of your code but with tools to help manage that copy and support re-integrating it with your main source tree if you want to), and being able to roll back to an earlier version if you decide you have take a wrong turn.
You might have seen Git before, perhaps as a plugin to an IDE or as a standalone GUI app, but Git is primarily a command line application. Even if you will mostly use Git through an IDE or GUI it can be very valuable to learn the command line version as this will give you an unfiltered view of the true power of the system.
I'd suggest starting with the getting started exercise, and then continuing from there with practise and additional background reading.
There are many resources that will help you to learn Git. A good place to start is the Git SCM book which is available online and to download in a variety of formats. Other resources and online tutorials include:
Using Git in combination with a site like GitHub or BitBucket is a good way to build a portfolio of code that you can use to demonstrate to potential employers and collaborators that you are a capable developer.
The goal for this week is to at least create one repository of your own, in your own github account (which you might have to set up/register), and to commit and push the C64 bat and ball game code listing from last week into it.