Silver is a soft currency used to improve lure cards and purchase single cards. Players win it during events, and buy it in the store.
There is a daily limit on how many hunts are rewarded with silver - after you reach it, you need to wait until you will be able to get silver for your hunts again. You can check when the limit resets by tapping the silver icon on the hunt summary.
The silver limit can be increased by upgrading the "Contracts" skill for every location.