Mindfulness means keeping your consciousness alive to the present
Thich Nhat Hanh defines mindfulness simply as keeping one's consciousness alert and present to whatever is actually happening, rather than letting attention run on autopilot while the mind is elsewhere. He illustrates this with the image of someone drinking tea while thinking about work, or driving while replaying an argument — technically present in body, absent in awareness, and therefore missing the actual experience entirely.
The definition matters because it strips mindfulness of exotic trappings; it isn't a special altered state reached only through advanced meditation, but a quality of ordinary attention that can be applied to literally anything, starting immediately. He suggests that most of a typical day passes in this half-present condition, and that mindfulness is simply the correction — noticing you've drifted, and returning.
*Takeaway: pick one routine activity today and notice, honestly, how much of it you actually experienced versus passed through on autopilot.