"The Nymph's Reply to the Shepherd" is exactly what it says it is: a poem from a girl to a boy who likes sheep, written in response to a poem said girl originally received from the aforementioned boy. Not very complicated, folks.
Straightforward though the title may be, it points out an interesting facet of literary culture during the 1600s. We've made somewhat of a big deal about the connection between "The Nymph's Reply" and Marlowe's poem, "The Passionate Shepherd." What we haven't made a big deal about (until now) is the fact that the kind of poetic dialogue that Marlowe and Ralegh have going on here is actually not that big of a deal. That's right—people wrote "replies" and "responses" to other people's poems all the time.
Part of the reason for this is that the poetry-writing community in early modern England was relatively small—the time, supplies, education, and connections needed to become a well-known poet were only available to the very wealthy or the extremely determined. Also, the smallness of that poetic community created something really special: the literary coterie. (A coterie is just a small group of people with a shared interest. For example, we belong to a coterie of kitty scarf knitters. What? Don't you judge us!)
If you were writing poems in 1600s, chances were that you had lots of friends and acquaintances who were also writing poems. You would write poems on the same subjects and get your other friends to pick which one they thought was best, you and your friends would send poems back and forth to each other in letters, and you might even write poems about how awesome your other friends' poems were. Ralegh was no exception. In addition to his obvious familiarity with Marlowe's work, Ralegh was friends with Edmund Spenser and Queen Elizabeth I (a poet herself), was referenced in some of Shakespeare's sonnets, and his travel writings were one of several guiding influences on The Tempest.
Think back on the title for a minute. Notice how, even though it never mentions Marlowe's name, everyone and their mom seems to know that Ralegh is writing in response to "The Passionate Shepherd"? That, Shmoopsters, is the beauty of the literary coterie. It is also what we call a very, very, very small world.