The poem is a heartfelt invitation by the speaker to a person whom he wishes to live with him. Like all passionate speakers, he doesn't rely on logic to convince this person. Instead, he appeals to the person's senses and appreciation of the beauty in nature. The speaker ends up making promises that, in a literal sense, he cannot possibly keep.
(I'm assuming, of course, that the speaker is male here, because he's called a shepherd and not a shepherdess in the title of the poem. The person being spoken to might be female or male. The word "gown" made me think first of a woman, but the word "kirtle," which I had to look up, can refer to two different pieces of clothing, one traditionally worn by men and another by women.)
So what, exactly, does the speaker promise? He starts off by talking about how they'll delight in the beautiful simplicity of nature -- sitting on rocks, watches shepherds feed their flocks, listening to birds' songs, etc. -- but he then goes on to talk about how he'll manipulate their environment to make it even more delightful, such as fashioning a bed as well as clothes out of beautiful, sweet-smelling plants. From somewhere, these already fabulous items will be further decorated with rare, expensive materials that (while they do occur in nature) are much harder for a shepherd to come by than rose petals and lambs' wool: gold, coral, amber, silver, and ivory.
The poem ends by repeating the invitation from the beginning.
Of course, the overall idea here is that the shepherd is trying to get the woman to love him. Here's a brief paraphrase:
Come be my love and we'll have all the pleasure that is possible out here in the fields and mountains.
We can watch the shepherds and sit by waterfalls and listen to the songs of birds
I'll make you a bed of roses and clothes (a kirtle is like a dress) out of flowers and a thousand posies (so here he's being outrageous -- he can't do this stuff. It's like promising someone the moon.)
I'll give you really nice clothes and shoes and such.
I'll give you all of this stuff and also a belt made of straw if you'll just be my love.
I'll give you amazingly good food -- fit for the gods -- served on silver plates on an ivory table.
I'll have the shepherds sing and dance for you if you'll just be my love.
So he's just promising her all kinds of crazy, exaggerated stuff if she will love him.