The green light is mentioned earlier as Nick sees Gatsby reaching with outstretched arms at the site of the ilght across the water. Gatsby ultimately views Daisy as an idea or object. The green light is a symbol of her closeness to him. His improbable rise from obscurity is nearly complete and the object he desires is nearly within his grasp. Later in the novel when he begins his affair with Daisy the novel will state that the green light has no meaning to him anymore because he finally has her. This foreshadows the ending of the novel for there is nowhere further for Gatsby to go.
There are several items of interest pertaining to this quote. First, Gatsby is admitting to Daisy that he not only knows where she lives but that he's been keeping tabs on her. He knows the frequency and duration of the light at the end of their dock. Secondly, we see the importance of the color green (one of the themes/motifs) in this novel. Gatsby finally ties together that the green light at Daisy's place has continued to symbolize his willingness to keep going towards her. And finally, hard core theme-ists will suggest that the mist currently shielding the light from Gatsby and Daisy should have shown Gatsby that his perfect dream wasn't perfect. The clouding of his important symbol (the green light) should have suggested to him that he wasn't meant to chase Daisy anymore.