Subscription

1. What more do you need explained? It will not let you remove a payment method that currently has an issue or a pending charge until said issue is resolved or payment completed.
2. Yes, you can continue to use the subscription until it expires.

1. If there’s an issue with the payment method you won’t be able to get rid of it until the issue is resolved. Once that happens, it can be removed, and it will be charged to your account balance if any is available.
2. Some services like Netflix for example no longer support being paid through Apple’s platform, so if something happened to your payment method, and it could not renew the subscription, then it will simply expire, and you won’t be able to start it up again through Apple.

Exactly what it says: It will end in a short amount of time.

Check that your payment method is valid and can be charged and that whatever service this is for, did not stop supporting payment over Apple's payment service.

When it ends, it will simply end. No further charges are made once it expires.