Reanimating another player is exactly how it sounds. You need to play online or locally for this achievement. Have your partner "die" and they'll go down. Unlike when you play solo if one person is standing while the other is down, you won't get the loading screen. Instead, they'll stay down until one of two things:
1. The second player goes down, and then both players will "respawn" in a new location
2. The second player returns to the downed player and picks them up. (Reanimate)
Picking them up does not require any special equipment. Just hold down the button prompt and they'll be revived, You only need to do this once to earn the achievement, and it will only work if you're the one who revives the other person. The trophy should pop right after they're on their feet.
NOTE: Be careful when entering and exiting locations with another player down. This can cause the downed player to glitch out and be unreachable. Your best bet on fixing it is to leave the game and try again, but this doesn't always work.