It's a Voice Line.
Now there is another submenu after Challenges, that allows you to customize your "social" abilities. You can select gestures and voice lines that your character can use in the game. Once you pick the ones you want to use, upen the squad/rebel order menu in the game, press a button appropriate to your system (R3 on the PS4, but you'll have info on the screen which button to press) and use a gesture/voice line the same way you would issue an order to your squad.
Yes, it's ridiculous, useless and having JUST a voice line as a reward for finishing that op THREE bloody times (including once with no HUD) is just a huge disappointmemt.
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