Shepherd is a gear perk that raises your general defense at the cost of slower movement. It suits a tanky, stand-and-fight style where you would rather soak damage than dodge it.
You take less damage from everything, but you move more slowly. That trade favors holding a position over running between covers.
Wear the Prophet's Vest in the chest slot. 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.
Good for heavy weapon builds and defending a spot. Skip it for stealth or fast flanking, where the lost speed hurts more than the defense helps.