To seal a book was to keep it closed with something that makes it impossible for anyone to read what was inside without breaking the seal. The angel is telling John not to keep the message a secret. Alternate translation: "Do not keep secret ... this book" (See: [[rc://en/ta/man/translate/figs-explicit]])
Here "words" refers to a message. See how you translated this in [Revelation 22:7](../22/07.md). Alternate translation: "the prophetic message of this book" (See: [[rc://en/ta/man/translate/figs-metonymy]])