A repository, or repo, is where your project and all related files will reside. Think of it as project-specific folder where you can store code, datasets, images and other files you may need. When working with GitHub it's important to distinguish b/w a local repo and a remote (or hosted) repo. The first refers to a repo stored on your computer. The second refers to a repo stored in a cloud-based code hosting service such as GitHub – all repos created within GitHub are remote repos. If you plan to collaborate with others using GitHub, then you must first create a remote repo. To create a remote repo, go to your GitHub account and click on the 'Repositories' button and then the 'New' button.
Tip: You can also add an existing local repo to GitHub; see steps 8 and 9 from this link.
If you're project doesn't already exist on your computer, then you'll have to clone it from GitHub. To clone a repo, find the "SSH clone URL" under the 'Code' button in your GitHub repo of interest. The repo can be cloned by typing git clone ssh_clone_url within your local directory of choice.