This is a typical Shakespeare sonnet glorifying his lady's beauty, using the conceit of looking back to past descriptions of beautiful people and arguing that those descriptions of beauty foreshadowed the beauty of his own lady.
In the first four lines, the poet reads "descriptions of the "fairest wights" (that is, good poets), creating beautiful poems whose subject is the "praise of ladies dead and lovely knights." The first line has the modern sense of "bygone ages that have been destroyed by time."
"Then, in the blazon of sweet beauty's best" uses the term for a shield on which a person's coat-of-arms is painted ("blazon") and here the poet lists a catalogue of beauty--"of hand," "of foot," "of lip," "of eye," "of brow"--as if these items appeared on the "blazon." The poet notes that these ancient writers would have extolled his lady's beauty.
The concept of foreshadowing is introduced with "So all their praises are but prophecies. . . all you prefiguring," and it's quite possible that the poet is making a veiled comparison of his lady to Christ by using the concepts of "prefiguring and "divining eyes," something he did in a similar vein in Sonnet 105. The sense of lines 11 and 12 is that the only reason the old writers could appreciate the poet's lady's beauty is that they a supernatural ability, summed up in "divining eyes," which means the ability to see something others cannot see.
The poet acknowledges in the couplet that he and other people in the poet's present time can "wonder" at his lady's beauty, but they do not have the skill to praise her as the old poets did who had "divining eyes."
This is, in essence, a fairly conventional sonnet praising the lady's beauty but using the interest conceit of the contrast between the ancient writers' almost supernatural ability to describe beauty contrasted with the poet's inability to describe his lady's beauty.