The Acquire command allows an infantry unit to take equipment (primarily ammunition but also rockets) which is stored in a vehicle or bunker. Most infantry carriers (halftracks and trucks) carry additional equipment in storage compartments.
In order to use Acquire, the infantry unit must be the inside a bunker or vehicle (as passengers). Thus, an infantry unit too large to enter a particular vehicle cannot acquire ammo there (try a small team if it is a squad that can be split). Also, the vehicle or bunker must be carrying something that can be acquired; most do, but it is possible to acquire away all the extra ammo that is being carried.
Acquire is an instant command: it is effective immediately. A pop-up window lists all available equipment that the unit is eligible to choose from. Clicking on an entry removes the equipment or ammo from the vehicle and places it into the inventory of the infantry unit. The infantry can dismount, or do any other command it could normally do normally in the same turn.
Note that there is no "unacquire" command, nor is there any way to share ammunition or weapons between infantry units beyond the programmed function of ammo carrying teams.