While in the tavern you can always click your chest in the corner and browse through your available chests. You can open any chest that either doesnt require a key or requires a key and you own it. If the chest requires a key to unlock it, you can find that key next to the chest.
Chests can be found in various places in Journey Online world. For example tower of heroes and adventures.
-Keys to higher tier chests can be primarily found in the field post patch 0.2.2
Opening them will give you rare items and sometimes items that have never been seen before in Journey Online!