Selective Focus allows you to make a subject stand out from the background or elements around it. When you press the shutter part way the camera focuses on a point a certain distance away. This depends on where you are focusing and how far away the subject is. This plane of focus is parallel to the plane of the sensor.
If you want the subject to stand out you need to set a low aperture to decrease the depth of field. So f 2.8 has a shallower depth of field than f 8, f16 or anything higher. Depth of field is how far in front of and behind the plane of focus that will be rendered sharp in the image. So if you want the subject to stand out you need a shallow depth of field or low f stop.
One final consideration is how far from the subject you are. The closer you get to the subject the easier it is to decrease the depth of field. So being inches from the subject as opposed to feet or being 5 feet as opposed to 10 feet away, will help decrease the depth of field.
The example of the iron fence has the focus on the center fence spike with an f stop of 3.2 and I’m relatively close to the subject about 1 foot away.
The next image has the focus on the first wine bottle with f stop of 2.8. This makes it stand out from the others behind it.
The final example uses selective focus to highlight the grapes and then lets the grape picker go blurry but still be recognizable. To achieve this effect I am inches away from the grapes but need a higher f stop of 11 to render enough detail in the background.