There are many definitions of mindfulness, and some good web sites on the topic. I take the word mindfulness to mean "calmly paying attention in the most effective way possible". Often this means attending, without judgement, to aspects of the present moment that may get missed (such as bodily and emotional states).
Being mindful helps psychological change to occur and builds resilience in people for the future. My own study, training and practice in mindfulness dates back more than thirty years and influences all aspects of my work.