What is what does it mean to letter in a sport?

Lettering in a sport, often referred to as earning a "letter," is an achievement signifying a certain level of accomplishment and participation on a high school or college athletic team.

Essentially, it's an award recognizing a student-athlete's significant contribution to their team. The requirements for lettering vary depending on the school, sport, and coach, but they generally involve meeting specific criteria related to:

Earning a letter is often a source of pride for student-athletes, and it allows them to receive a varsity letter (a physical letter, usually the school's initial) to display on a letterman jacket or sweater. While the exact criteria are set by each school and coach, the overall idea is to reward commitment and athletic achievement. (https://www.wikiwhat.page/kavramlar/varsity%20letter)