It is a state of awareness and conscience during which your attention is focused on the present moment in a calm, clear, lucid and objective. We all experience mindfulness state and most of the time in a furtive way, some seconds or some minutes at the most, when we are relaxed and concentrated, looking at a chimney fire for instance, or during a walk in a bucolic environment.
Meditation allows people to reach this state of mindfulness. It trains your brain to cast stress and negative waves off in order to be able to focus more on the current time longer and deeper. The benefits you can get from training in mindfulness meditation practices are that significant and these practices are that powerful that it is often used to cure mental diseases, and in particular diseases related with stress handling. Broadly speaking, everybody who intend to practice mindfulness meditation directly feel a mental calmness and a better control over their feelings, obviously during meditation practice but also in their daily lives.
For beginners, a meditation session can last only some few minutes. The more advanced adepts of meditation can stay until hours in a state of meditation. The most important thing in favor of meditation is to practice it regularly: many sessions of few minutes but often are better than long sessions just sometimes and in an irregular way.