When signing up for our free 7-day trial, you will be asked to enter your payment information (via the website) or confirm the purchase using your Apple ID/Google Play password/touch ID/face recognition. This ensures that there will be no interruption in your service at the end of the trial period.
As stated at the time of purchase - when your trial ends, your account will automatically be converted to a paid subscription and you will be charged the subscription fee. Remember that you may cancel the trial anytime within the trial period without the risk of being charged.
Note that for payment verification purposes only, you may see a small charge on your billing statement during the trial (this is an authorization for future charges and not an actual charge, to make sure that the payment method you used is valid).