Mortality is a fancy word for the inevitability of death. "To the Virgins" talks about the death of a flower, the setting of the sun (another kind of death, and a metaphor for human life), about how getting older means getting closer to death, and about the possibility of a sort of living death, exemplified in the poem's nightmarish vision of an unmarried life. Even though death is everywhere, we can still make the most of what time we have.
Although the speaker emphasizes the brevity of life, and thus the importance of acting during one's "prime," both the sun (stanza 2) and the flowers (stanza 1) suggest the possibility of rebirth or second chances. The rosebush will grow new flowers, and the sun will rise again.
"To the Virgins" describes both literal deaths (the flower) and a number of figurative ones (the setting of the sun; the "tarrying" of the last stanza), and it remains undecided about which is worse.