Also: Contemplation, Awareness
Mindfulness is focused awareness of the present moment. Mindfulness lets us be fully conscious of a simple sensation like the warmth of sunlight or of the complex interplay between our thoughts and feelings.
By tuning in to mental processes, we are able to recognize that our thoughts are just thoughts; they don't necessarily represent reality. We can observe them rather than being subject to them.
Mindfulness lets us absorb the richness of the moment instead of going through life with half of our attention on the past or future or our own mental chatter. The self-knowledge that comes from mindfulness lets us be more intentional in choosing priorities and actions that fit our life mission.