The main theme of the poem is about the ravages of time as used by Shakespeare and many other poets in their sonnets.
Here the narrator is advising the virgins to make use of their youth whilst they still have it on their side. He warns them that if they don't heed to his advice their condition might be like the flowers who smile brightly when they bloom , but it's only ephemeral because by the time they they soak themselves in their beauty, it's time for them to fade away or wither.
In the second stanza the narrator uses the metaphor of Sun to convey the theme of passing of time. He says, as the glorious lamp of heaven, the "Sun" when in his full energey is at the highest point, fades away when it's time for him to finish his race.
In the third stanza the narrator is convinced that the best time for everybody to enjoy is "youth" when your blood is warm i.e. when you have beauty and energy on your side. Once they leave your side you will only face the worst time of your life and nothing less.
The last stanza is a kind of didacticism, wherein the narrator advises the coy virgins not to waste their prime time because they won't get it back once they have lost it. It's no use repenting afterwards about the things they could have done in their youth once they have outgrown it because of their coyness.