Indicated airspeed
This is the number you use for many aircraft operating limits and performance references because the airplane’s aerodynamic behavior is tied closely to the pressure sensed by the pitot-static system.
True airspeed
As altitude and atmospheric conditions change, true airspeed can differ from indicated airspeed. This matters when thinking about navigation and performance.
Groundspeed
Groundspeed is how fast your position changes over the ground. Wind can make it higher or lower than true airspeed.
Simple mental modelIAS = what the airplane feels through the air; TAS = how fast it is moving through the air; GS = how fast it is moving over the ground.