Has swaps inside it. Can also contain a devil or TNT.
Bronze has 12 swaps and item find is 12%
Silver has 70 swaps and item find is 33%
Gold has 400 swaps and item find is 100%
There is nothing after gold.
Some people like to save up their chests for gold chests. You should only do that if you have plenty of room to work with.
After using your last swap for the day you can quickly tap a chest to get more swaps to stop the day from ending.
Swaps are basically labor to pay workers to move things around. When you run out of swaps there's nothing else you can do so the day ends automatically without you having to press a button to say that you're done for the day. So instead the game gives you 1 second to open any chests. If you open a treasure chest you're basically paying workers overtime for extra labor for the day.