A certificate lets you issue and update Apple Wallet passes under your own brand. You manage certificates under Settings, which you reach from the profile menu.
A certificate serves two technical purposes. It signs the pass so the Wallet app can check the file is intact and that you are allowed to issue it. It also secures the connection to the Apple Push Notification Service, which lets Apple decide whether to send a push and which pass to update.
Use a new certificate when the pass has its own purpose, for example a different company, or when it should use a different push icon.
Holders can no longer save the pass, and you cannot deliver push notifications. We email you well before the expiration date so you have time to update it. Once you update the certificate, everything works again.