What's he saying?
"Then let not winter's ragged hand deface, / In thee thy summer, ere thou be distilled:"
Don't completely lose the youth within you before you bear a child:
"Make sweet some vial; treasure thou some place / With beauty's treasure ere it be self-killed."
Save that youth and hold it dear before it dies with you.
"That use is not forbidden usury, / Which happies those that pay the willing loan;"
A loan that you enter into willingly and that brings you happiness is not wrong;
"That's for thy self to breed another thee, / Or ten times happier, be it ten for one;"
So have a child, or even better, have ten;
"Ten times thy self were happier than thou art, / If ten of thine ten times refigured thee:"
If you had ten children, you would be ten times happier than you are:
"Then what could death do if thou shouldst depart, / Leaving thee living in posterity?"
Then you would beat death, because even after you die, there would be ten more of you still living.
"Be not self-willed, for thou art much too fair / To be death's conquest and make worms thine heir."
So don't be obstinate, because you're too beautiful to let your beauty die with your own body.
Why is he saying it?
Sonnet 6 is a continuation of sonnet 5; both are "fair lord sonnets," which are either addressed directly to or written about the effect of a young and strikingly beautiful man. They are also both "procreation sonnets," which focus on the fair lord's responsibility to have a child so that his beauty might be passed on for future generations to appreciate. While Sonnet 5 was about aging in general, with no direct address to the fair lord, in Sonnet 6 the speaker expands the metaphor and makes it clear that he is calling upon the young man to bear children.
The distillation of flowers involved preserving their perfume in a glass vial, before being used in cosmetics and confectionaries. This process is used as a metaphor in both Sonnet 5 and Sonnet 6 for having children, and thus preserving part of one's self. In line 3 of Sonnet 6, the speaker urges the fair to "Make sweet some vial," referring to the vial in which a perfume would be stored. It is necessary to have children "ere thou be distilled," or before age overtakes him.
In this case, the "vial" is a metaphor for a woman's womb, which would bear the fair lord's child. Semen was thought to be the essence from which a life was made, while a woman was just the vessel for development. So in telling the fair lord to "Treasure thou some place / With beauty's treasure," the speaker is instructing him to impregnate a woman's womb with his sperm.
Winter is personified in the first two lines: "Then let not winter's ragged hand deface / In thee thy summer, ere thou be distilled:" continuing the metaphor from Sonnet 5. The young lord must have children before "winter's ragged hand" can take his youth from him. The "ragged hand" refers to the rags that winter in human form was often depicted as wearing. The word "deface," in addition to meaning "ruin," hints at the particular changes that old age brings to a face, namely wrinkles.
The final couplet of this sonnet hints at masturbation; the speaker thinks it is a waste, since it won't result in children for the fair lord. "Self-willed," which hearkens back to "self-killed" of line 4, is a sexual innuendo. The various sexual meanings of the word "will" are played out in Sonnets 135 and 136. "Death's conquest" refers to one who has died, of course, but the idea of dying was a common euphemism for having an orgasm; each orgasm was thought of as a little death. So being "death's conquest" also means not using one's orgasms wisely, or wasting semen that could be used to breed children.