Chests are awarded randomly as long as there's a free spot available in the chest tray on the right hand side. Higher bets make chests drop more frequently.
Awarded chests take time to unlock. Silver chests take 3 hours and gold chests take 8 hours. It's a good idea to start the unlock of the gold chests at night so they're ready to open the next morning!
Opening a chest will award you with stickers. Stickers be used to match onto quest cards in order to complete quests that give extra coins rewards.
Keep an eye out for rare chests such as platinum, emerald and diamond chests! They're very rate and take longer to unlock, but are more valuable.