What exactly does the +1 Force Charge mean on the Fifth Brother crew card?

Fifth Bro, like all other gunners & crew with Force effects, add one recurring Force charge to their ship.

Unlike regular charges, upgrades do not have their own individual Force charges, but rather contribute to the total pool of Force charges available to the ship. If you put Vader crew and Fifth Bro gunner on a Decimator, that Decimator would have two Force charges, and you could spend them to power either Vader or Bro, or to modify dice normally.

A ship with multiple sources of Force charges still only recovers one Force charge during each turn's end phase.

To cite the source of Maui, Rules Reference pg 7 under Charge .

Quote

* Each card with a Force capacity (the purple number) starts the game with a number of [force charges] equal to the Force capacity. Each [force] starts on its active side.

* If an upgrade card has a Force capacity, this increases the Force capacity of the ship. The [force charges] are placed above the ship card it is attached to (not the upgrade card).

* If an upgrade card instructs the ship to spend [force charges], those [force charges] are spent from the ship card.