It is used to indicate that the submodule branch should be the same as
the branch of the parent repository - devel for devel, master for master.
It's a new feature of git, appearing in version 2.10, but most users
should not really care. It only affects the behavior of "git submodule
update --remote" when updating the submodules.
The git protocol is so limited that GitHub doesn't show it when selecting "Clone
of download". It may be blocked by strict firewalls, whereas https would work
almost certainly. Pushing code is not allowed over git protocol, but it's
possible over https with a tocken.
