Mindfulness is defined in many ways. As there are many ways to define mindfulness, there are many ways to practice.
One is the ability to see things as they truly are; not as you wish them to be. When you see things as they are instead of as you would like, you can make better decisions for your own happiness, and for the happiness of others. It's called practicing mindfulness because when you practice you builds up your ability to be able to be present in everyday life and see things as they truly are in each moment.