The depth of field plays a very important role in photography. It is controlled by f-number of the lens. Here,a large aperture, which means small f-number is selected. As a result of it, we get shallow depth of field. This is ideal for focusing only the required object, without focusing the back ground.
Try taking this kind of photo. Put f-number at 4 and capture a big flower like the one shown. You can easily see that only some portion of the flower are in clear focus. By doing this, you can emphasize required portion of the flower.You can clearly see that in the above photo, the back ground is not in focus and also, the pollen grains are out of focus.