When you sign up for a subscription to a product from Avira, you have the option to select manual or automatic renewal. If you choose the convenience of automatic renewal, your subscription is billed about 35 days before it expires. This way, if there is any issue with the payment, you have plenty of time to take care of it before losing Avira coverage.
This doesn’t affect how long your coverage lasts in any way. You still get the same amount of coverage that you paid for, you just don’t have to worry about paying for your subscription renewals.
You can find more information on Avira’s website here.