The poem's main theme is love and its effect on the lover. The speaker is so enthralled by his beloved that he can't imagine life without her.
To be without her is to experience "nothingness". In other words, his existence and his experience of life has become tied to her.
He then describes her impact on him by using strong visual imagery: "slicing the noon like a blue flower", "the light you carry in your hand, golden".
That image, the one about the light in her hand, may be an allusion to Midas, who turned everything he touched to gold.
Unlike the Midas tale, however ,this has nothing to do with greed or materialism.
The poet is simply saying that she redeems everything around her. She makes life beautiful.
In the lines "which maybe others will not see ... like the red beginnings of a rose" he shows that her effect on him is personal. There's nothing grand about her, nothing that would win her the admiration of the world. He alone sees what makes her special and he loves her for it.
In the third verse, he compares her arrival in his life to a "gust of a rosebush", and a "wheat of wind". The images, which are very original, have fresh and pleasant connotations. He is also showing how quick and unexpected the meeting was.
Since that meeting, they have become one and their lives are inextricably linked.
The last line expresses his belief that their love will last and grow: "And through love I will be, you will be, we'll be".
Neruda's poetry often has an organic, sensual feel to it. And this one is no exception. He achieves this by his use of imagery from nature: "rosebush", "wheat of wind", "blue flower", etc. and by his use of simple, but profound language. He seems to get to the essence of things, their inner beauty, while ignoring their form or outer appearance. When he describes, for example, the light that she carries in her hand as "the red beginnings of a rose" we can feel the fragile beauty and intimacy of it.