When you apply for a school, you often need to get "letters of recommendation". These are letters written by a teacher, boss, coach, or some other authority. They're supposed to be about how great you are. The letters are usually sealed in a closed envelope and sent along with the application.
Letters of recommendation are commonly needed for getting into college, graduate school, getting a scholarship, or for winning certain awards. They're not usually needed in the U.S. for getting most jobs.