What? Mindfulness is the practice of paying attention that helps us consciously live in the present. It is practiced in silence, paying attention to breathing, senses, emotions, sounds and more.
Why?  Because we almost always live on autopilot, distracted by multitasking and suffer our thoughts without understanding that they come and go so we should not  identify with them. Mindfulness allows us to grasp our negative thought patterns before they overwhelm us and suck us into a downward spiral. It gives us back control of our lives.
How? Learning to pay attention intentionally, in the present moment, with open mind and without judgment.
When? In any moment of life, in fact you can practice at any time of the day, any place, any situation.