Trim Speed is the speed to which a vehicle, such as a glider or boat will settle in the absence of control inputs. "Trim" alone refers to the left/right balance of a vehicle (see trim tab).
Trim Speed is one of the cardinal speeds which it is important to know when flying a glider of any kind. Trim, as it is also known, is the speed at which the aircraft will fly without pitch inputs (in still air).

Trim speed depends partly on the location of the aircraft's center of gravity relative to the center of lift of the lift producing surfaces, and partly on the load the aircraft is carrying. In hang gliders, trim speed is adjusted by setting the location of the hang strap (hang point) from which the pilot's weight is suspended (from the keel). Forward equals faster, aft slower. A lighter pilot will have a faster trim speed than a heavier pilot on the same hang glider, all else being equal. This is because the heavier load causes the airframe to flex more, so the wingtips produce less lift, raising the nose and slowing the aircraft.
For a typical hang glider, stall speed is 18-20 mph, minimum sink is about 21-22, trim speed is set to about 22-24, and best glide is somewhere in the 25-30 mph range. When the trim speed is set in this range it takes minimal effort to maintain either minimum sink or best glide, and flying hands off will lead to neither a dive nor a stall. Pilots who race hang gliders sometimes set their time speed at or near best glide, to reduce fatigue flying at that speed (as they are wont), but their gliders then require more effort to fly slowly in lift and are harder to land because they trim so far above stall speed. Devices to allow trim speed adjustment have had sproadic interest over the years in hang glider racing, but have never caught on.