Soft-Target Defender is a gear perk that raises your defense against both soft-target and armor-piercing rounds. It covers two of the main enemy ammo types at once, which makes it a strong general pick.
Hits from soft-target and armor-piercing ammo do less damage to you. Many FND soldiers use one of the two, so you feel it in most fights.
Wear the Soft-Target Vest MK II, Space Marine Plate or The Duke. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Yes for most builds. It is one of the broader defense perks you can put in the chest slot.