A Shakespeare sonnet 35. I don't get it!?

a sonnet is a poem, properly expressive of a single, complete thought, idea, or sentiment, of 14 lines, usually in iambic pentameter, with rhymes arranged according to one of certain definite schemes, being in the strict or Italian form divided into a major group of 8 lines (the octave) followed by a minor group of 6 lines (the sestet), and in a common English form into 3 quatrains followed by a couplet.