Certificates are issued by third parties to certify some of your information. It can certify the validity of your passport, your age, your address or even simply the fact that you are registered with a service.
When someone sends you a certificate, you are free to keep it or archive it.
Before this decision, you can review the details of the certificate.
Once a certificate is archived, it can not be used to register with services.
Most of the time, when registering with new services, you need to attach certificates that will help you get on-boarded faster.