A cryptographic proof that a web page existed, verified by independent nodes.
In HTTPS, you and the server share a secret key. You could show someone the data you received, but you could also have made it up. There is no way for a third party to know.
Most services that offer web page proofs run their own verification server. You have to trust them. debust is different.
Each debust also takes a full-page screenshot. The screenshot plus the cryptographic proof together give you a solid record of what was on a page and when.
The proof uses TLSNotary, a protocol for proving what a server sent during a TLS session.