In the non-electronic world, hand written signatures are used to verify the authenticity of the sender of a message. In a electronic world, it would be hard to use such a signature and it would be easily forged. A digital certificate is based on the properties of asymmetric or public key cryptography. The certificate has to be renewed after this period, or a new one has to be requested. This is achieved by using the person's public key to encrypt the message.
Confidentiality is assured because only the intended recipient can decrypt the message.
Thus a certificate can be used to identify a person named in a certificate, if that person owns a private key that belongs together with the public key in the certificate. Of any received signed mail we can be sure that it can only be generated by the named person because the attached certificate was signed by a trusted third party. For example paying for goods that we order at a web shop with a Visa card.