Chapped meant "jawed" in seventeenth-century English. "Slow-chapped" therefore means slow-jawed. Marvell's meaning becomes clearer if we look at what he says in context:
Rather at once our time devour
Than languish in his [time's] slow-chapped power.
The poem depicts the lovers as in a race and battle with time. Time is personified in these lines as a powerful person with jaws. These jaws will slowly chew up the lovers until they are old and dead. It is better, the poem's speaker argues, to "devour," or quickly eat up, time in pleasure rather than to let time eat them up. If they make love now, time loses and they win.
The speaker plays on the speed with which mortal life passes. Time is a commodity that humans have a very short supply of.
The poem expresses the theme of "carpe diem," or "seize the day." This was a common theme in poetry at the time, expressing the idea that time is fleeting and that we should enjoy our pleasures while we still can.
Marvell wrote the poem in 1681, near the end of a century of upheaval, turmoil, and civil war. The monarchy had been abolished; the monarchy had been restored. Against this backdrop, life seemed insecure and uncertain. Better not to rely on time, the poem asserts, when we never know what will happen next.