When you use the command:
git pull origin
"origin" is the name of your remote repository that you're specifying in that pull statement.
Take a look at your .git/config file. You'll see something like this:
[remote "origin"]
fetch = ...
url = ...
If you have multiple remotes, then you'd be able to specify which one to pull from. For example, you might have a remote "origin" repository on a backup hard drive that you push to, and when you're ready to release to github, you like to squash first, then push there. You'd have two remotes listed in your .git/config file - one for your backup drive, and one for github.