Someone who's retired has stopped working permanently. A retired teacher who misses his job might volunteer at an after-school program.
Most retired people in the U.S. are over the age of 65, although some are able to retire earlier, and some continue working well into their 70s or 80s. Some jobs provide pensions, or partial salaries, to retired workers, and other retired people take advantage of government-provided benefits like Social Security. To retire means "to stop working," but also "to retreat or withdraw," and in the 1500s, retired meant "separated or withdrawn from society."