You will probably work on a project that requires dependency on another project. This can be a library that was developed by you or another team. It can be hard to manage when the library is updated and you made some custom code inside your project.
Git handles this by using submodules. It allows you to manage a Git repository as a subfolder of another Git repository, which in turn lets you clone a repository isolated from the commits of the current repository.
Let's imagine you are working on a website and you want to add the
fpdf library that helps you create a PDF file in PHP. The first thing to do is to clone the library's Git repository inside your subfolder:
Erik@server:~/mySite/$ git submodule add ...