The Wenyan (文言, "commentary on the words" or "patterned words") is one of the commentaries in the Ten Wings, and the only one devoted to just two hexagrams: Activity and Passivity. Tradition sees them as the gateways of the book, pure yang and pure yin, and so they get this extra treatment.
What it says
It opens by reading the four characters of the judgment of "Activity," 元亨利貞, as four qualities of character (the four virtues). Put briefly: origin is the first of goodness, success is the meeting of what is excellent, benefit is the harmony of what is right, and constancy is the trunk that holds every task up.
Then it goes line by line, often as questions and answers with "the Master said." The hidden dragon of the first line becomes someone of real virtue who doesn't chase a name and isn't troubled at going unseen.
Why it matters
The Wenyan shows how a divination book came to be read as a book of ethics. For the noble person, the lines stop being omens and become advice for living.
Origins: tradition and scholarship
Tradition credits it to Confucius. Scholars date it later and point out something curious: the reading of the four virtues appears almost word for word in the Zuozhuan, spoken by a noblewoman, Mu Jiang. That suggests the Wenyan gathers teachings that were already in circulation.