Airliners—since the retirement of the Concorde—fly below Mach 1, the speed of sound. But at high subsonic speeds the air over the top of the wings can reach or exceed Mach 1. This can result in the formation of shock waves that destabilize the pressure distribution around the aircraft, before the aircraft itself has exceeded the speed of sound. In order to stay within the aerodynamic design limitations that this imposes, airliners are typically engineered to cruise between Mach .78 and Mach .86.
Pay attention to Mach even on subsonic jets (all commercial now obviously) because air over wing traveling faster than aircraft and if that air moving over wing does that, can cause shockwaves and destabilize. So they're usually below Mach .85
