A Chest is an object that will spawn after selling certain Cars during a Special Event. Depending on the level of the event you are currently in, it will take a certain amount of Work Points to open the chest and spawn golden Mystery Cards, Coins and or Donuts. Some workers also have Strong+ stats on Chests.
Item Chests can be bought from the Menu and contain different number of items depending on the Cash value. Item Chests drop Booster, Skill+ items, Mystery Cards and Golden Mystery cards. The Mystery cards can contain anywhere from Booster Items to Cash rewards. Sometimes opening a chest results in an in-game email which gives another free Chest containing 10 Mystery cards. You might also get an email with discounted prices for Cash. In a rare chance a chest may also contain a Green Devil worker.
Note: Be careful when collecting items from chests. If you touch the screen before all the items have popped out you probably won't get all the items. Let all the items come out of the chest then touch to collect them all.