The brachial plexus which innervates the whole arm is sandwiched between the anterior and middle scalenes and the first rib.
If any of your ribs/vertebrae/etc don't articulate right and/or get stuck then the muscles around them tend to get tight and painful in some cases.
Basically, he's trying to make sure the first rib is mobilized and not stuck because it can lead to some arm issues.
Nothing that new or too awesome but most people could get some relief from that