Mindfulness is the practice whereby a person is intentionally aware of his or her thoughts and actions in the present moment, non-judgmentally. Largely associated with Buddhism, in which it is called Sati, the practice of mindfulness is also advocated by such people as medical researcher and author Dr. Jon Kabat-Zinn, psychologist Nathaniel Branden and philosopher Ayn Rand. Psychologist Ellen J. Langer has written several books on what she calls "Mindfulness," (including a book with that title), which popularize the experimental work she's done on the topic. She acknowledges similarities between the "Mindfulness" she investigates experimentally and the Eastern concepts like sati, referred to in English by the same name, but her research does not draw specifically on Buddhist or any similar tradition, and the practices she advocates for attaining mindfulness are not exactly the same.
The simplest, but also fundamental, example of sati is to mentally give a verbal label to each inbreath and outbreath during sitting Meditation. So, each time one breathes in, one thinks (e.g.) "rising", and each time one breathes out, one thinks "falling". In this type of meditation, the breath serves as a tether that the practitioner uses to bring his or her awareness back to the present moment. By residing more frequently in the present moment, practitioners believe that they are less influenced by the endless stream of thoughts, memories and wishes that characterize "mindless" existence.
Another example, this time not confined to sitting meditation: during a given day, one commits to mentally giving a verbal label to the position of the body whenever the position of the body changes. So, when one is sitting, one thinks to oneself "sitting", without actually saying it out loud but just imagining saying it. Then, when one gets up, one thinks "standing up", then after having gotten up one internally verbalizes "standing". If one starts walking, then one thinks "walking", and at the end of the day if one lies down then one thinks "lying down".
Eventually mindfulness is applied to not just to bodily actions but also to the mind's own thoughts and feelings. In Buddhism, this kind of mindfulness is considered a prerequisite for developing insight (Vipassana) and wisdom (Panna).Yoanjii