git push assumes that you already have a remote repository defined for that
branch. In this case, the default remote origin is used.
git push origin master indicates that you are pushing to a specific remote,
in this case, origin.
This would only matter if you created multiple remote repositories in your code
base. If you're only committing to one remote repository (in this case, just
your GitHub repository), then there isn't any difference between the two.

//push all branches to origin
git push origin

//push master branch to origin
git push origin master