Mindfulness training is about developing intimate engagement with reality, as it unfolds moment-by-moment, regardless of whether we perceive reality to be pleasant, unpleasant or neutral. While this may seem like a simple concept, it requires much patient training. Yet stabilizing in awareness, even for brief periods, is to enjoy a freedom and quality of life that makes the practice abundantly worthwhile.
A nice 2014 summary of the neuroscientific understanding about how Mindfulness works from Scientific American: http://blogs.scientificamerican.com/guest-blog/2014/06/12/what-does-mindfulness-meditation-do-to-your-brain/
Ritchie Davidson PhD and Jon Kabat-Zinn PhD concisely summarize some of the neuroscientific data in this (6 minute) 2012 YouTube video: