From Mindfulness in Plain English by Bhante Henepola Gunaratana, Chapter 13 ("Mindfulness (Sati)"), the central meaning of mindfulness:

Mindfulness reminds you of what you are supposed to be doing.

In meditation, you put your attention on one item. When your mind wanders from this focus, it is Mindfulness that reminds you that your mind is wandering and what you are supposed to be doing. It is Mindfulness that brings your mind back to the object of meditation. All of this occurs instantaneously and without internal dialogue. Mindfulness is not thinking. Repeated practice in meditation establishes this function as a mental habit which then carries over into the rest of your life. A serious meditator pays bare attention to occurrences all the time, day in, day out, whether formally sitting in meditation or not. This is a very lofty ideal towards which those who meditate may be working for a period of years or even decades. Our habit of getting stuck in thought is years old, and that habit will hang on in the most tenacious manner. The only way out is to be equally persistent in the cultivation of constant Mindfulness. When Mindfulness is present, you will notice when you become stuck in your thought patterns. It is that very noticing which allows you to back out of the thought process and free yourself from it. Mindfulness then returns your attention to its proper focus. If you are meditating at that moment, then your focus will be the formal object of meditation. If you are not in formal meditation, it will be just a pure application of bare attention itself, just a pure noticing of whatever comes up without getting involved — "Ah, this comes up ... and now this, and now this ... and now this".

Mindfulness is at one and the same time both bare attention itself and the function of reminding us to pay bare attention if we have ceased to do so. Bare attention is noticing. It re-establishes itself simply by noticing that it has not been present. As soon as you are noticing that you have not been noticing, then by definition you are noticing and then you are back again to paying bare attention.

Mindfulness creates its own distinct feeling in consciousness. It has a flavor — a light, clear, energetic flavor. Conscious thought is heavy by comparison, ponderous and picky. But here again, these are just words. Your own practice will show you the difference. Then you will probably come up with your own words and the words used here will become superfluous. Remember, practice is the thing.

(cf. Present-Moment Reality (2008-11-05), Notice and Return (2013-03-11), Bare Attention (2016-06-20), ...) - ^z - 2016-07-15