configure a remote for fork

To sync the fork with origin source, you should set a remote origin.

git remote -v
origin  https://github.com/YOUR_USERNAME/YOUR_FORK.git (fetch)
origin  https://github.com/YOUR_USERNAME/YOUR_FORK.git (push)

git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git

git remote -v
origin    https://github.com/YOUR_USERNAME/YOUR_FORK.git (fetch)
origin    https://github.com/YOUR_USERNAME/YOUR_FORK.git (push)
upstream  https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git (fetch)
upstream  https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git (push)

original source : https://help.github.com/articles/configuring-a-remote-for-a-fork/