The prize chest is a chest of rewards you receive for completing the Path to Glory event. What is special about the prize chest is that rewards from it can be claimed more than once. You can find the list of rewards in the prize chest info window.
How do I get rewards from the prize chest?
Complete the Path to Glory event at least 8 hours before the event ends. Continue saving up glory coins by completing levels in restaurants where plot stars are available. Once the prize chest meter is full, you can claim your rewards.