When you start your free trial, your subscription provider (e.g., Apple, Google, HBO, etc.) verifies your payment method by placing a temporary authorization hold on your payment method (the amount varies by provider). This is done to ensure your payment method is valid and can be successfully charged once your free trial is over.
The authorization hold is not a charge but may affect your available balance. The time it takes to remove an authorization hold varies by financial institution. You can contact your financial institution to find out when the authorization hold will be removed and when the hold amount will be added back to your available balance.