Cloning a repository means making a copy of a repository on GitHub, to download and work on locally--on your local machine.
Cloning can be especially useful when you're joining a group project that is hosted on GitHub, and you want your changes to eventually be pushed and shared with that same repository.
Cloning can also be used in conjunction with forking. For example, you could fork the GitHub repository of this lesson to your GitHub account, and then clone or download the repository to make edits and admendations. Then you would push those changes to a repository under your GtiHub account. Git and GitHub work to make sure attribution of contributions and lineage are transparent.