A digital ID is a certificate that you can include in your email messages. It helps people who receive your email know it came from you, rather than from someone else making their messages appear to come from you. Digital IDs can also be used with some email apps to encrypt messages, which hide message contents and indicates if a message has been changed.
If you want to use a digital ID with your work account, check with your system admin. For personal use, you can get a digital ID from a certification authority (CA) such as VeriSign, Thawte, or GlobalSign.