i am staring at this card and just not getting it.

"Each card you control in reserve is counted as a command icon at its planet when resolving command struggles."

what does it mean for a card to be in reserve? does this card need to be exhausted in order to use it or is it just a passive power that's always on after it is deployed at HQ?

so i deploy Beleaguered Garrison in my HQ and then this just applies to all the planets at which i have units at and they count their hammers and themselves for command points?

"Reserve" means cards with the Deep Strike keyword that have been deployed face-down at a planet, but not flipped over by paying the Deep Strike cost yet.

So, if you have deployed this support card at your HQ, all of your face-down (i.e., otherwise blank) Deep Strike cards anywhere will count as if they had 1 command icon during command struggles.