By definition, mindfulness means:

1. The quality or state of being conscious or aware of something.

2. A mental state achieved by focusing one's awareness on the present moment, while calmly acknowledging and accepting one's feelings, thoughts, and bodily sensations, used as a therapeutic technique.
In golf, if we keep thinking about a mistake we made on a previous hole, or project how our score will turn out at the end of the round, our minds become filled with images of the past or future. It’s as if we’re daydreaming. Lacking mindfulness of the task at hand leads to poor decision-making before the shot and distracts us while we’re swinging.