If you spend vast amounts of time, each day, worrying about the future or the past, you are not fully aware of the present, and it is gone before you know it. Focus all of your attention on what you are doing right now, and there is no room for anxiety about the future of concerns about the past. That's mindfulness.
Strange, isn't it? You didn't read a book which ran to hundreds of pages, or pay large amounts of money for a personal course, and yet you now understand mindfulness.