Why use origin in the push?

Hi there! When you first made your repository you might remember doing a command like git remote add origin <link to repo>. At that point, you set up a sort of pointer to your remote repository named "origin". There is the possibility that at some point, however unlikely, that you could have multiple remote repositories (potentially for backup purposes). Each "remote" would need to be named something. Whenever you would then push to a remote branch it needs to know which remote branch it should push to. In this case, it's stating that you want to push your gh-pages branch to the remote repository that you previously aliased as "origin". To be clear, you can name that anything you like. In fact, I once spent about 3 hours pulling my hair out because I was not able to push only to discover that I had originally named my remote "orign" instead of "origin". Note that they differ by one letter.

Hope this helps!