Roses, seas, rocks, sun. There's a whole lotta shakin'—oops, we mean nature going on in this poem. The speaker uses nature in various ways to describe the depth and power of his love. But hey, this poem comes from rural Scotland—the land of lochs and glens and heaths. Is it any wonder this guy would use nature to write about his love?
Lines 1-2: The speaker compares his love to a red, red rose. And because he uses the word "like," this is a simile.
Lines 7-8: The speaker says he will love his bonnie lass until the seas dry up. The evaporation of the "seas" appears to be a metaphorfor the end of the world or for something that can't ever really happen. So really he's just avowing his undying, eternal, everlasting (and other cheesy things) love for his special friend.
Lines 9-10: The speaker mentions the seas going dry again, and adds that he will also love his "bonnie lass" until the "rocks melt w' the sun." Melting rocks are also a metaphor for the end of the world, or for something that isn't likely to happen.