You can focus in two ways. One way is to focus on a specific point. Just find a point in front of you and look at it intensely. You will notice that everything, but the point becomes blurred.  This is just like a camera. When you focus on a specific point with your camera, everything apart from the point will be blurred. This comes in handy in golf. You do not want to be burdened with other golfers attempts to put you off focus. Moreover, you do not want to be confused with all kinds of hurdles on the golf course. Simply visualise your ball flight and roll. That will make sure you do not focus on sand, lakes, and other anxiety trickers. This is one of the tricks to avoid anxiety.  

Perimeter focus is the other way to focus. Once again you look on a point, but this time you FOCUS on the perimeter and note both what you see, hear, smell, and feel. It is a completely different set of data you collect.