Carpe diem is a Latin phrase that is often translated as "seize the day," although many serious Latin scholars might argue that something like "pluck the day" would be more accurate. In any case, the general contemporary meaning of the phrase is "enjoy the present," and it is essentially an exhortation to take advantage of life, as one will not go on living indefinitely.
This phrase is important in regard to Andrew Marvell's "To His Coy Mistress." In general, the poem involves a speaker who is trying to convince his love (who is being "coy") to make the most of her youth and love him passionately in the moment. Basically, the speaker argues that they don't have all the time in the world, that mortality will catch up with them, and that they shouldn't therefore wait any longer to fully express their love for one another. As such, one might easily argue that carpe diem is central to this poem, as it communicates the main idea of Marvell's piece: make the most of the present, live in the moment, and live life to the fullest while one can.