Metaphysical poetry is poetry that contains references to what reality, or the idea of being, really is. It is philosophical (and often religious) poetry which contemplates what human beings and the universe are. While Marvell certainly put metaphysical ideas in this poem, this is not the main thrust of it. This is a "carpe diem" poem (Latin for "seize the day"), along the lines of Robert Herrick's earlier poem "To Virgins, to Make Much of Time" (and many other poems in the seventeenth century) which encourage young people, specifically reticent maidens, to grasp physical love and/or marriage while they may. While Marvell gives his argument a metaphysical slant by beginning with "Had we but world enough and time".
Marvell imagines that if he and the lady he is wooing had all of eternity (note the references to huge expanses of time: "ten years before the Flood" (line 8), "Till the conversion of the Jews" (10), "and hundred years" (13) and even "thirty thousand" (16) years, and "An age at least" (17)) they might indefinitely put off the consummation of their love. In creating this world of unlimited time -- in fact, of the possession of eternity, as God would have -- Marvell can use hyperbole to its highest degree. Each of his lady's charms would be worth endless years of praising, he said, if they had the time. His point is that they do not.
He moves on to a more pessimistic thought: while the lady's charms are worth no end of praising, and no amount of time spent wooing would be in vain for her inestimable worth, there is only a limited time allotted to the lovers before they are taken to the "marble vault" (26), and "grave" (31). So now Herrick's blunt statement of "And while ye may, go marry" (Herrick, line 14) is summed up in Marvell's gentler, more persuasive poetry. We must not muse on the niceties of courtship, Marvell says to his beloved, and contemplate the eternity of our feelings and the excellence of each other; we must act.
Marvell ends the poem by saying that they should enjoy their union while they are still young, and he ends with a celestial metaphor (common to many metaphysical poets). "Thus we cannot make our sun/Stand still, yet we can make him run." In the face of the undeniable fact of their eventual (and, from how the poet talks, perhaps imminent) deaths, they must fiercely "tear our pleasures with rough strife" (43) -- that is, burn up their sunny days with love. While this poem retains the metaphysical flavor of many of Marvell's poems, it is largely one of love and seduction, as most "carpe diem" poems are.
Source: Renaissance Poetry. Leonard Dean, ed. 2nd ed. Englewood Cliffs, New Jersey: Prentice Hall, Inc., 1961, pp 303-4.