Treasure chests are a staple of Dungeons, and the Dicey Dungeons are no exception. Walking to them and opening them allows you to take what's inside for yourself.
If playing as the Witch or the 6th character, you can choose to ignore what's inside and come back for it later.
Be careful when opening chests - one of them could be a Mimic that you'll have to fight.
When the Mimic rule is active, everything in the dungeon will have the treasure chest sprite instead of its normal sprite.