What does command 'git rebase' mean when no arguments followed?

Taken from the man page (git help rebase):

   git rebase [-i | --interactive] [options] [--exec <cmd>] [--onto <newbase>]
           [<upstream>] [<branch>]
   git rebase [-i | --interactive] [options] [--exec <cmd>] [--onto <newbase>]
           --root [<branch>]
   git rebase --continue | --skip | --abort | --edit-todo

   If <branch> is specified, git rebase will perform an automatic git
   checkout <branch> before doing anything else. Otherwise it remains on
   the current branch.

   If <upstream> is not specified, the upstream configured in
   branch.<name>.remote and branch.<name>.merge options will be used; see
   git-config(1) for details. If you are currently not on any branch or if
   the current branch does not have a configured upstream, the rebase will
   abort.

This means that git rebase, alone, defaults to git rebase branch.<name>.remote branch.<name>, as long as there already exists a configured upstream for this branch - otherwise, it aborts.

If you only give it one argument, it will be the name of the upstream branch, while keeping you in the same branch you were.