We know from the title that this poem is a response, or reply, to someone and/or something else that's already been written—in this case Christopher Marlowe's poem, "The Passionate Shepherd to his Love." Reading Marlowe's poem is pretty essential to understanding what Ralegh is doing with his response, but even without having read it we still get the gist of what "The Passionate Shepherd" is about by reading Ralegh's first stanza. (Hint: Ralegh's answer is not promising for the shepherd in question.)
Whereas Marlowe's lyric begins with optimistic (and ultimately idealistic) promises, Ralegh's poem begins with a conditional statement that sets up a rejection of Marlowe's shepherd's proposal: I might be persuaded to come live with you if a) I didn't have real world problems to think about and b) you had been telling the truth about what our life together would be like, but I do and you weren't so see ya… never.
The next three stanzas seal the deal, listing the ways and reasons the shepherd's promises are too good to be true, like the fact that flowers he promised will wither and die, or that it will eventually get too cold for them to hang out with the sheep by the river. These might seem like odd reasons to turn down a potential love interest, but here is where being familiar with Marlowe's poem is really useful: each stanza in Ralegh's verse dismantles, dismisses, or qualifies a promise made in the corresponding stanza of Marlowe's original. Nifty, eh?
By the time the final stanza arrives, the nymph seems to have basically crushed the poor shepherd's dreams. The poem, however, has a slightly more optimistic conclusion than you might have expected. The nymph's final words to the shepherd imply that, were some of the more humdrum realities of life removed (like the fact that we all get old and die and that somewhere in between now and then we have to pay for like, 200,000 meals and maybe some clothes), perhaps her answer would be different. It's a moot point because the situation she describes is unobtainable, but that doesn't mean those final lines aren't worth a good, close look.