What does the term "certified mail" mean and how is it used in sending important documents or packages?

"Certified mail" is a service offered by postal services that provides proof of mailing and delivery for important documents or packages. When sending something via certified mail, the sender receives a receipt and a tracking number to confirm that the item was sent and delivered to the intended recipient. This service is often used for legal or important documents to ensure they reach their destination securely and on time.