The Treasure Chest is a very rare item obtained from fishing.
Its only use is to sell it, which grants the player 5,000 gold. Not much else is known about this, although if you talk to Elliot in the library, there is a chance that he will say something about a pirate ship which sunk near Stardew Valley long ago, and a treasure that had been lost. This treasure can be caught in a river or ocean and may be what Elliot is talking about. However, there is still the possibility of catching the Treasure Chest from fishing in rivers, lakes, and caves.