According to the speaker in Shakespeare's Sonnet 106, ancient writers were more skilled than the Renaissance poets in expressing the beauty of youth. The ancient writers produced pictures of the "fairest wights," or most lovely people, people who are now spirits because they have died. Medieval writers made "beautiful rhymes" about ladies and knights who are also now dead. The writers from these various times in the past captured the beauty of youth at its height in "hand ... foot ... lip ...eye ... [and] brow."
However, the speaker says, all they really did was foretell how beautiful his beloved is today. They looked with "divining" eyes—eyes that could see into the future—otherwise they would have been unable to write of beauty they way they do, as it is all embodied in a person alive after they are long dead.
The irony is, the speaker says, that the ancients had the poetic words to describe someone as beautiful as the beloved, but the beloved wasn't alive then. Ironically, too, the people who are able to behold her beauty today can look at it with "wonder" but lack the language to convey it.
The ancient writers were the best at extolling beauty. If only, the speaker says, they could have seen his beloved: their words would have done this person justice.