What's he saying?
"Full many a glorious morning have I seen / Flatter the mountain tops with sovereign eye,"
I have seen many beautiful mornings make the mountains look more beautiful than they are,
"Kissing with golden face the meadows green, / Gilding pale streams with heavenly alchemy;"
Making the green meadows and the pale streams appear gold;
"Anon permit the basest clouds to ride / With ugly rack on his celestial face,"
But soon ugly clouds overtake the sky,
"And from the forlorn world his visage hide, / Stealing unseen to west with this disgrace:"
Hiding the sky as morning becomes night:
"Even so my sun one early morn did shine, / With all triumphant splendour on my brow;"
In this way, the fair lord used to bless me with his presence;
"But out, alack, he was but one hour mine, / The region cloud hath mask'd him from me now."
But that was only for a short time, and now he is gone.
"Yet him for this my love no whit disdaineth; / Suns of the world may stain when heaven's sun staineth."
But that does not weaken my love for him one bit, since if the sun in the sky can sometimes be overcast, so can my beloved.
Why is he saying it?
While the poet has been focused on his own mortality in Sonnets 27-32, in Sonnet 33 it is clear that his attitude toward the fair lord has changed drastically. The fair lord has rejected the speaker, and the speaker's negative attitude is conveyed through his choice of diction. He uses the words "ugly" and "basest," in stark contrast to the beautiful, heavenly character he has created of the fair lord in previous sonnets. This focus on being hurt by the fair lord is extended through Sonnets 34 and 35, as well.
The morning is personified as a king in the first four lines of Sonnet 33. The use of the word "sovereign" calls a ruler to mind, as well as the term "flatter;" however, if the sun is the king and the mountains his courtiers, the role of flattery has been reversed. The morning and the sun become the same character through the term "sovereign eye;" the sun is like the eye of the sky, and through the idea of "kissing," which the sun seems to do to the meadows.
Imagery of alchemy pervades this sonnet; alchemy was perceived to be part science, part magic, and involved turning base metals into gold. It involved trickery, and thus is fitting for describing the betrayal by the fair lord that the poet feels he has suffered. In line 4, the glorious morning is described as "Gilding pale streams with heavenly alchemy." To "gild" something means to cover it with gold; in this case, the sun is performing a kind of "heavenly alchemy" by seeming to transform the water of the pale streams into gold. But in line 5, "basest clouds" overtake the sky; the word "base" is a reference to dull metals.
The final couplet can be read as a return to the previous devotion the poet had for the fair lord; though he has been rejected, his love does not falter. However, it can also be read with sexual implications, especially since the word "stain" implies some impurity, perhaps that of a sexually transmitted disease. In that case, the final couplet can be taken to mean that the fair lord has contracted a disease, and will pass it on to the "suns of the world" with whom he has sexual contact.
It is also likely that the "disgrace" suffered by the fair lord is the same disease. Though he used to shine brilliantly, now his face is obscured by an "ugly rack" of clouds. This idea is enforced by the use of the word "stealing" to describe the now overcast sun's movement across the sky; it implies that the fair lord has been engaging in illicit sex, and thus contracted a disease. That disease is the "region cloud" that now hides the fair lord's beauty from the poet.
