A 'certificate' is the term designated to the 5-digit PIN that is used in order to access the Digidentity Wallet app.
Unfortunately these certificates are not valid for an indefinite period and will need to be renewed. If your certificate expires without being renewed, you will no longer be able to access or use the service in which you have registered for.
When you complete the registration for your eSignature Qualified, you will be provided with a 'Qualified certificate'. This will be stated under your name listed 'Log In' section of the Digidentity Wallet app. Your Qualified certificate will be valid for 1 year after its creation.
When your certificate is coming up to it's expiry date you will be notified via your email address. You should be sent emails on day 30, 14, 7, 2 and 1 prior to your certificate expiring.
The expiry reminder email will provide you with steps to follow in order to renew your certificate, see an example below: