The extension of your upgrade plan. You can buy and activate an upgrade plan at any time and it will just add 12 more months to your upgrade plan.
For example, suppose your current upgrade plan is active until Sep 1, 2019 and you buy an upgrade plan today for €129 (they’re having a sale), then your upgrade plan will be active until Sep 1, 2020.
There’s no monthly fee; that’s just what you end up paying roughly to stay on their upgrade plan if you renew your upgrade plan ahead of time whenever they’re having a sale.