Most cameras enable you to either select one single point of focus or choose to allow the camera to pick the point of focus.
By using auto area focus mode the camera will make the decision as to what should be in focus. Your camera might even have modes to focus on faces. When using auto area the camera will assess everything in the scene and will choose what it thinks is the most important to focus on. As you half press the shutter button dots will appear in the viewfinder to show you what the camera has chosen to focus on.
By selecting single point autofocus you choose the exact point in the photo that will be in focus. This puts more control back in your hands. You can choose to have that point in the centre of the frame or towards the edge. When you half press the shutter button you will see a single point illuminate in the view finder. This is the single point that you have chosen and you can move this point onto whatever you want to focus on, either by moving the camera or moving the focus point using the directional pad on the back of the camera. The camera will always focus on where you place the focus point in this mode.