Mindfulness is a mental state where you have a calm, accepting awareness of your body and mind. It has its roots in Buddhist meditation, but by no means does it need to be spiritual: it can be just a way of being aware of, and accepting, your thoughts. There's increasing evidence to suggest that mindfulness is very useful in coping with lots of different problems - like depression, anxiety, pain and addictions.
It's not a quick fix, and it takes practice, but it can have a huge effect on your ability to let go of your negative thoughts and routines. The ultimate goal of mindfulness is freedom, although happiness and relaxation can be welcome side-effects.