What is the difference between TAS and GS?
TAS = True Airspeed = speed that you get on radar gun as airplane flies by, when radar gun is held by someone in gondola of balloon in same airmass (wind motion) as airplane.

GS =Groundspeed = speed that you get on radar gun as airplane flies by, when radar gun is held by someone on ground.

As an example: TAS of 200 knots and a headwind of 20 knots gives a GS of 200-20=180 knots.  That is: the plane travels at 180 knots over ground but the air is flowing past the plane at 200 knots.