We could summarize this poem in four words: this guy's in love. But we'll give you a little more, just so you know what to expect.
The speaker is addressing his lover, and begins by saying that he doesn't love her in the way that someone would love beautiful flowers or gems. Great; so how does he love her? Well, first, he loves her in secret. Scandalous! He also loves her as if she were a flower that wasn't in bloom, which is full of beauty on the inside. And in fact, that flower (his lover) produces an aroma that he carries inside of him. A rather interesting metaphor, don't you think?
After spending the first eight lines trying to describe his love, Neruda uses the remaining six lines to raise the white flag: he can't explain his love, he just plain loves this girl. His love is simple and humble. The speaker ends with the image of him and his lover, intertwined, as if they were one body and one soul. He may not be able to define his love, but his attempts sure are evocative.