There's three parts to this command:
git push
This invokes the push command
origin
This names the remote to which you are pushing. This is either one of the named remotes stored in .git/config (you can list these with git remote), a URL, or the token . which means the current repository.
master:refs/heads/master
This is called a "refspec", and you can read about it in the man page for git push. But in general, it's comprised of two parts, separated by a colon. The first part is the name of a local branch, and the second part is the name of a branch on the remote repository (in this case, origin). This particular refspec could be shortened to master:master.
In general, one can shorten refspecs even further. Just specifying master as the refspec is equivalent to using the same name on the remote, so master is the same as master:master.