Mindfulness is the process of bringing awareness to the present moment and experiences, internal or external. More commonly used within the idea of Mindfulness Meditation it’s referencing a type of meditative practice based on Buddhist traditions. It has been brought to western countries in recent decades and considered it to be introduced by John Kabat-Zinn.
Researchers have investigated Mindfulness to be effective to reduce stress, pain, anxiety as well as gaining focus on necessary tasks. Clinicians may use it with patients who suffer from this or related difficulties. The lay people may practice it nowadays thanks to the extension of meditation exercises, workshops or books available out there.
While the use of Mindfulness practices may be a great addition to any kind of treatment, I would not recommend to use it in place of any medical treatment.