Allied to da Vinci's use of sfumato is his focus on the Mona Lisa herself, rather than the background landscape, which would traditionally have been delineated in a similar degree of detail as the sitter in a portrait. If you look at the lowest areas of the picture plane, at the edges and at the landscape, you'll see that da Vinci uses shadows to highlight his subject, making her considerably more real, more full of life, than her immediate natural environment.
The light in which the Mona Lisa herself is bathed emerges from the darkness of the shadows. Observe how her hands emerge from the blurred shadows of her body, focusing the viewer's attention on these areas of light. There is no stark contrast here; light emerges from darkness and remains in fruitful tension with it, creating a suitably mysterious paradox to go along with the sitter's famously enigmatic smile.
To create the smile sometimes described as "enigmatic", da Vinci employed a technique known as "sfumato", where outlines are blurred and the use of soft coloring and thin, translucent glazing allows forms to merge one into another creating a soft, sometimes mysterious or ethereal effect, in da Vinci's words, "in the manner of smoke without lines or borders". This technique was also used to create his subject's gauze-like veil, scarcely visible at first view. Da Vinci's active study of human anatomy didn't hurt his execution of this portrait, either, as is particularly noteworthy in the woman's hands.
The opposite of this, "chiaroscuro" is the use of paint to create the illusion of lighter parts emerging from darker areas, a technique that creates drama, and allows an artist to create a three-dimensional, sculptural effect on flat canvas. Use of this effect is seen in the hints of a rounded figure, the subject's skin juxtaposed against dark, heavy clothing draped around her.