Git is a version control system (VCS), which is just what it sounds like: a system to help keep track of different versions of software. Git isn’t the only VCS out there (others include CVS, SVN, and Fossil), but it is one of the more popular systems, particularly for open source projects. You’ve certainly used software that was developed using Git (Firefox and Chrome are two big ones!).
Version control is really helpful when you are working with other people. Without version control, if I send you a file I’m working on and you make changes to it, we would suddenly have two versions. If I integrate your changes into my file, then we’d only have one file but no history! Even when working alone, version control is really helpful for us to keep track of how the project is moving along.
Understanding at least one source control platform is vital for software development. Git can be like pulling teeth (and then there are the times when it gets really painful), but if you are developing software (even personal scripts!) and don’t have source control in place, you’re walking a tightrope without a net.