means a booking code airlines use to price and ration seats on one flight, so economy is actually many sub-fares stacked like nesting dolls.

from emerged from airline revenue management in the 1980s, after deregulation let carriers charge whatever the market would bear; the fare class letter (Y, B, M, Q, etc.) became the industry's secret handshake for how much wiggle room, mileage, and refund rights your ticket actually carries.