What does lettering in a sport mean?

Lettering in a sport usually has to do with your High School years. If you letter in a sport that means that you played on the Varsity team and you remained on the team for that entire year. To letter you must play a varsity sport and usually at the end of the year most teams throw a banquet and your coach will hand you the initials of your school which is considered your letter, it is also what you put on letterman jackets in high school.