What is a letter jacket?

Here’s some context (the term is actually at the end of the text):

"My freshman year of high school, I transferred to a public school from the private one I’d attended since kindergarten. I was excited about it. Feeling a bit mischievous, I decided to embark on an experiment in my new environment. Instead of telling people that my name was Priscilla, I thought I’d just introduce myself by another name, a nickname I’d made up for myself.
My childhood friend Nicole was one of the few people I knew who went to this school, and I brought her in on my plan. She wasn’t too crazy about it, but she agreed to play along. So on the first day, each time she’d stop to introduce me to a group of her friends, she’d turn to me and say, “This is Pri—I mean, uh,” her voice trailing off for a moment. She’d shoot me an inquisitive look to make sure I wanted this charade to continue. Yes, I smiled in response. Yes, I did.
Before the day was out, I’d been officially stamped by my new nickname. In fact, there wasn’t a single teacher, student, or member of the school administration who did not refer to me by that name when I graduated four years later. I even had it monogrammed on my sports uniforms and my letter jacket."