upstream in the github example is just the name they've chosen to refer to that repository. You may choose any that you like when using git remote add. Depending on what you select for this name, your git pull usage will change. For example, if you use:
git remote add upstream git://github.com/somename/original-project.git
then you would use this to pull changes:
git pull upstream master
But, if you choose origin for the name of the remote repo, your commands would be:
To name the remote repo in your local config: git remote add origin git://github.com/somename/original-project.git
And to pull: git pull origin master
Source: Stackoverflow.com