When contributing to a specific repository, you will clone it. That's your little playground now to experiment. But your changes and your repo itself will not be synced up to the main repository you forked from. Here's how to keep it up to date while experimenting with your changes.
- Create a remote in your forked/cloned repository pointing to the main repository
git remote add upstream [URL of main repository you forked from]
- Fetch the upstream changes
git fetch upstream
- Merge upstream changes with your forked/cloned repository
git merge upstream/master