The "Let Them Theory," popularized by Mel Robbins, is a mindset shift to stop trying to control or change other people, freeing up your energy by accepting their choices and behaviors, even if you don't like them. It involves pausing, saying "Let them," and then focusing on what you can control (your reaction, thoughts, actions) through the "Let Me" principle, fostering peace and better boundaries by allowing people to be themselves, which reveals their true nature for your own clarity.