What does \"sealed\" mean?

A sealed call has been appended to the ledger with a timestamp and a hash, and can't be edited afterwards. Once that day's fingerprint is published, any later change to the call would be detectable by anyone holding the published fingerprint.

Details

1. Appended: the call is added to the ledger when it's logged. Proof of Call users will get a receipt with its call id and hash (planned).
2. Day sealed: after the UTC day ends, the day is closed. Nothing can be recorded on or before a sealed day.
3. Anchored: the day's fingerprint (head) is published to the public ledger-heads repository on GitHub.

"Sealed" means tamper-evident, not tamper-proof: changes are detectable, not impossible. It shows the call existed in the ledger and hasn't changed since. It doesn't show the call was good, or that it's everything the writer said. See What doesn't the ledger prove?