In many situations, people need to certify that a document existed on a certain date. Think about a copyright or patent dispute: The party that produces the earliest copy of the disputed work wins the case. With paper documents, notaries can sign and lawyers can safeguard copies. If a dispute arises, the notary or the lawyer testifies that the letter existed on a certain date.
In the digital world, it's far more complicated. There is no way to examine a digital document for signs of tampering. It can be copied and modified endlessly without anyone being the wiser. It's trivial to change the date stamp on a computer file. No one can look at a digital document and say: “Yes, this document was created before November 4, 1952.”
This protocol uses Trent, who has a trusted timestamping service, and Alice, who wishes to timestamp a document.