The ghillie suit is a specially camouflaged outfit, designed to blend in with the foliage of Appalachia.
The suit is unlocked through progression in the Nuclear Winter mode during the Survivors event and is unlocked at 14650 total Overseer XP, after completing the final 2500 XP challenge. After unlocking it, it can be crafted at any armor workbench.
The ghillie suit offers no bonus to stealth. It is purely cosmetic.