Sonnet 5 compares nature's four seasons with the stages of the young man's life. Although the seasons are cyclical, his life is linear, and hours become tyrants that oppress him because he cannot escape time's grasp. Time might "frame / The lovely gaze where every eye doth dwell," meaning that everyone notices the youth's beauty, but time's "never-resting" progress ensures that this beauty will eventually fade.
In an extended metaphor, the poet argues that because flowers provide perfume to console people during the winter, it is natural for the youth to have a child to console him during his old age. Without perfume from summer's flowers, people would not remember previous summers during the long, hard winters; childless, the young man will grow old alone and have nothing to remind him of his younger days.
Winter, an image of old age, is regarded with horror: "Sap checked with frost and lusty leaves quite gone, / Beauty o'ersnowed and bareness everywhere." The "lusty leaves" imagery recalls the "lusty days" from Sonnet 2 and reemphasizes the barrenness of the youth's old age, in which he will look back longingly on his younger days but have nothing to remember them by. However, in the final couplet, the poet evokes a comforting tone, suggesting that immortality is attainable for the young man, just as it is for summer's flowers when they are transformed into perfume, if only the young man would father a child.
distill(ed) reduced to the essence.