Git is a version control system: a tool to manage your source code history. During development the code changes a lot. Also, multiple front-end developers often work on the same code. In order to improve the workflow, tracking changes to code and enable collaboration, end developers make use of Git.
Git allows developers to easily collaborate, as they can download a new version of the code, make changes, and upload the newest revision. Git then keeps track of the changes and make sure that every developer works on the correct version of the code.
There are several hosting services for Git repositories, like Bitbucket, GitHub, GitLab. With these services you can host and review code, manage projects, and build software alongside other developers.