Definition
Upper-body restraint straps that pass over the occupant's shoulders and attach to the seat belt system, holding the torso against the seat to prevent the upper body from pitching forward during sudden deceleration, turbulence, or impact.
Plain English
Straps that go over your shoulders and clip into your seat belt to keep your upper body from being thrown forward if the aircraft jolts hard or crashes.
Context Anchor
In an inadvertent thunderstorm encounter, pilots are advised to secure shoulder harnesses because strong turbulence can move the airplane abruptly.
Derivation
Harness originally referred to equipment fitted around an animal or person to hold, guide, or control movement. That idea helps here: shoulder harnesses hold the upper body in place so it does not move freely during abrupt aircraft motion.
Why Pilots Care
They keep the pilot in position and in control of the aircraft during sudden turbulence or abrupt maneuvers.
Intuition Check
Do not think of shoulder harnesses as comfort straps. In this context, they are safety restraints meant to keep your body controlled during sudden aircraft movement.
Example Sentence 1
Before entering the area of suspected thunderstorm activity, the pilot tightened the seat belt and shoulder harness and briefed the passengers to do the same.
Example Sentence 2
Even with strong turbulence the shoulder harnesses kept the crew securely in their seats.