Choose an arena. This turn, when any of your units in that arena would be damaged, prevent 1 of that damage.
If a Battle card (such as Unfriendly Fire) or ability (such as the one on Rebel Armored Freerunner) damages more than one of your units at once, Deflectors Activated prevents 1 damage to each of the units.
If you play 2 Deflectors Activated cards, the effects add up. This means you prevent 2 damage each time any of your units in the chosen arena takes damage.