The overall theme of this sonnet by Henry Howard is the incomparable beauty and virtue of his lady.
In stanza one, Howard tells other poets not to bother extolling their ladies' beauty because his lady's beauty not only beats every other lady's beauty, but comparing her beauty to that of other ladies is like comparing the sun to the light of a candle or day to night--in other words, there is no comparison.
Stanza two focuses on the lady's virtues, which are so numerous that Howard doesn't even have the skill to describe them. In fact, his lady is as honest as Penelope the fair, and you can trust what she says as if it were written and sealed (in other words, you can take her word as if it were a written commitment).
Howard recounts in stanza three that he heard Nature herself complain, because Nature, having created the lady, lost the ability to create another like her and was inconsolable at the loss of the "perfit mould" (that is, the perfect model of a virtuous, beautiful lady).
Stanza four again recounts how Nature felt that, having already created the perfect lady, the inability to create another like her was a loss so great that Nature felt it in the core of her being ("near her heart"). Howard's last couplet reiterates that Nature's principal complaint was that she couldn't make another lady like this one again.