Blast Defender is a gear perk that improves your defense against explosions and soft-target rounds. It covers grenades, rockets and a common FND ammo type in one slot.
Explosions and soft-target bullets both do less damage to you. That helps against grenade-heavy squads and launcher troops.
Wear one of the gear pieces that carry it: Impact Vest MK II. Each piece only takes one body slot, so plan around the other perks you want.
Yes for loud play around vehicles and heavy troops. Two defense types in one slot is good value.