Simple living was the practice of simplifying one's lifestyle.
In 2267, a spore infected Elias Sandoval described his settlement on the planet Omicron Ceti III as having a simple philosophy where people returned to a less complicated life with few mechanical things, no vehicles, and no weapons. They had harmony and complete peace. (TOS: "This Side of Paradise")
That same year, Greek god Apollo demanded loyalty, tribute, and worship from the crew of the USS Enterprise, and in exchange he would grant them life in paradise as simple and as pleasureful as it was thousands of years before on Earth. (TOS: "Who Mourns for Adonais?")
According to the Star Trek Encyclopedia, 4th ed., vol. 1, p. 296, Mahatma Gandhi strived to improve the quality of life for the lowest classes of society, preaching the benefits of manual labor and simple living."