What is meant by these criteria for a stabilized approach?
Although I can’t find regulations regarding stabilized approaches in the FAR/AIM, the unofficial but generally accepted definition of a stabilized approach would be

“Only small changes in heading/pitch.
How small?”
Small changes in heading and pitch would mean changes only necessary to maintain a ground track on the lateral guidance and a consistent airspeed on the glideslope. These are to compensate for drift due to things such as wind.

“The aircraft is in correct landing configuration.
What are those configurations?“
Configuration is entirely dependent on the aircraft and situation.
Typically it would include things like:
Autopilot correctly set or off;
Boost pump on;
Carb heat to hot;
Gas on the fullest tank, both, or on;
Landing gear down and locked with indicator lights lit;
Cowl flaps closed;
Flaps at the correct setting;
Mixture set at best power for density altitude;
Prop speed set at fastest setting;
Landing, position, and anti-collision lights (if not in clouds or fog) on;
All auxiliary and/or unnecessary equipment off;
Sterile cockpit;
Seats and seatback set and locked in the upright position;
Seatbelts and shoulder harnesses on and latched;
Throttle/power set at a consistent setting to provide a descent rate on the glideslope;
Landing checklists complete.

“The aircraft speed is not more than Vref+20 or less Vref.
Is this IAS or CAS or TAS?“
Vref is usually in IAS. Especially since the occasion in which you are using it would be below 3000 feet AGL, and less than 200 knots.