The chest is a structure that allows players to store an infinite amount of one particular item. Chests generally become useful mid to late-game, when players need many different resources, but don't have the inventory space to carry them. Chests are also used for item exchange / trading between players.
If you take the item from a chest (including your own) you fail all quests that you have not already completed. The exceptions are "Open 5 treasure chests" and "Be peaceful".
The chest was added in the Community Experience update, and was suggested by Manfredini. As of the He is Alive update, the chest is now compatible with the lock and key, to protect their contents from being taken by non-team members and remove locks, respectively.
Lock and Key
Players can use locks and keys to protect a chest and raid locked chests, respectively. When the chest is locked, players can still break it and any items inside will disappear, just like a normal chest. To remove a lock, a key is required. When a key is used on a lock, both the lock and the key are destroyed, granting free reign over the chest. The person that locked the chest, and any of their team-mates can access the chest without keys.
Chests were added in the 'Community Experience' update.