There are three things that affect Depth of Field (how in focus something is from foreground to background.) Your aperture setting (F-stop,) the focal length of your lens, and the distance you are away from your subject.
1. Aperture: the larger the hole (ie: f/2, f/4) the shallower the depth of field (less in focus from foreground to background). The smaller the hole (ie: f/16, f/22) the wider the depth of field (more in focus from foreground to background)
2. Focal Length: A wide angel lens will produce imagery with more in focus from foreground to background such as a 35mm, 14mm, or lower. A telephoto lens will produce imagery with less in focus from foreground to background such as a 200mm, 400mm, or higher.
3. Distance: The closer you are to your subject the shallower your depth of field will be. The farther away you are from your subject the wider your depth of field will be.
Why is this a “tip” or a “trick?” Most point-and-shoot cameras are set to have the most in focus from foreground to background. It is difficult to manipulate a consumer camera to obtain shallow depth of field. It is a dead give-away that something was shot with a higher end camera when things are out of focus in the background or foreground. Shallow depth of field creates a more beautiful look to your imagery and your they will feel more professional.