Stands for Mindfulness Based Stress Reduction. It is an eight week course which was developed by Jon Kabbat Zinn,

Mindfulness-based stress reduction (MBSR) is a program that incorporates mindfulness to assist people with pain and a range of conditions and life issues that were initially difficult to treat in a hospital setting. Developed at the University of Massachusetts Medical Center in the 1970s by Professor Jon Kabat-Zinn, MBSR uses a combination of mindfulness meditation, body awareness, and yoga to help people become more mindful. In recent years, meditation has been the subject of controlled clinical research. This suggests it may have beneficial effects, including stress reduction, relaxation, and improvements to quality of life, but that it does not help prevent or cure disease. While MBSR has its roots in spiritual teachings, the program itself is secular.

Text is taken from Wikipedia, accessed Dec 31, 2017. https://en.wikipedia.org/wiki/Mindfulness-based_stress_reduction.