Mindfulness is living in the now, not the past or future.
“Mindfulness” is the common modern term applied to the practice of experiencing here and now. Those who aren’t mindful may very well spend an entire day alternatively ruminating about the past and planning for the future, never appreciating what they are experiencing in the moment.
In the book ”How to Do Nothing” this concept is pitched as quite literally ‘doing nothing’, which is what enables you to ground sense the concrete world around you wherever you are.