The looter achievement is obtained by opening 50 chests or containers. Once achieved, a player will be 20% of the way towards the Master Looter achievement.
Master Looter Achievement
Loot 250 containers.
In order to open a container and receive its contents, it must not have been opened by another player. Unopened containers can be identified by a sparkling effect above the container.
Unopened containers can typically be found in bunkers and dungeons that haven't yet been explored by other players.
Looting containers like crates, barrels, and sacks count towards this achievement.