Mindfulness is a way of paying attention to our experience. It has qualities of curiosity, alertness, allowing and patience.
Mindfulness isn't a particular state such as relaxation or peacefulness. Its more about how we are relating to these states. We can be mindful of tension and anger as much as we can more pleasant states but it can be harder to not resist or get "sucked into" these states.
Mindfulness is a faculty that can be trained and developed over time. We may find that some days we are more mindful than other.
Mindfulness meditation is the formal practice of mindfulness.