A payment has failed

When you and your customers' receive a "A payment has failed" email, this means that the card they're using to pay for your subscription has not been charged due to various reasons. These could be

The customer's account will be automatically cancelled after 3 failed payments. The timing of these attempts can be set in your Stripe dashboard with full documentation here - https://stripe.com/docs/billing/revenue-recovery/smart-retries

Your customer can change their own card details by going to Your Account -> Payment Plans -> Full Information

This will take them directly to their Stripe dashboard where they can update their payment details or pay an invoice that's in arrears.