Sealed documents were common in Bible times. Isaiah had already used this figure in chapter 8:16. Here the same illustration is used to show that because of Israel’s blindness of heart, no one was able (or willing) to accept and understand the will of God (see 6:9-10).
And the vision of all this has become to you like the words of a book that is sealed. When men give it to one who can read, saying, “Read this,” he says, “I cannot, for it is sealed.” And when they give the book to one who cannot read, saying, “Read this,” he says, “I cannot read.”(Isaish 29:11-12 ESV)
The photo below was taken in the archaeological museum in Gaziantep, Turkey. It shows a rolled up document with three strings held by clay seals. It appears that the document is modern with three ancient seals, but it illustrates what Isaiah is writing about.
The illiterate man says, “I cannot read.” The man who is literate says, “I cannot, for it is sealed.” The illiterate man does not try to find someone who can read, and the literate man does not find someone with authority to break the seal. Blindness, darkness, and spiritual insensitivity prevent either man from finding out what is in the document. No wonder Jesus cited Isaiah’s statement to explain why he used parables (Matthew 13:13-15).
The IVP Bible Background Commentary: Old Testament (Walton, Matthews & Chavalas) has this explanatory note about sealed documents in Bible times.
Official documents were written on scrolls of papyrus or vellum and then, when stored or dispatched by messenger, were rolled up and sealed with string and an affixed seal (see 1 Kings 21:8; Jer 32:10–11). The seal, either a ring or signet, was impressed into either wax or a lump of clay known as a bulla (Job 38:14). Archaeologists have found many of these clay bullae with the names of Israelite officials.
I am aware of one ancient document with as many as seven seals. It is the Wadi ed-Daliyeh Aramaic papyrus document dating to the 4th century B.C. The seals are currently displayed in the Israel Museum, Jerusalem. This document provides an illustration for Revelation 5 and 6.