The poet continues on to bid his love goodbye, and we see in this last stanza why he has been asserting his love in such strong terms. He is leaving, and he seems to want to convince his lady friend of his love before he departs. He does promise her that he will be back, even if he has to travel 10,000 miles. This bit of hyperbole (10,000 miles is about 1/3 of the whole earth and certainly a great deal of area to cover in Burns' day!) is the final touch in his profession of love.
This poem is about the poet's love for a young girl (a bonnie lass). He uses two main similes to explain the beauty of his love. Keep in mind that these similes have become stale now because they have been said thousands of times. But at the time of this poem, they were fresher. He says that his love is like a rose (beautiful) that's "newly sprung" (fresh). He also says that it is like a "melody" or song that is "sweetly played" or beautiful. So he starts the poem with a declaration of the beauty of his new love. It is also interesting that he uses a simile with something from the natural world (roses) as well as something made by people (songs).
He goes on to state that he will love this young woman "till a' the seas gang dry" or till the seas are dried up. That's a long time. So even though his love is like a rose, it will surely last longer than roses, which are known to fade. Not content to just assert that he will love her till the seas are dry, the poet continues on to say that he will love her till rocks are melted by the sun. As this is not a common occurance, this could be taken to mean that his love will last forever.
Please see the next post for more.