Speeds in aviation can get confusing. There's Indicated Airspeed, Calibrated Airspeed, True Airspeed, and Ground Speed (and Mach Number but we won't go there).
Indicated Airspeed is a measure of the Impact Pressure (also called Dynamic Pressure) of the air as you move through it. Impact Pressure is the force exerted by the air as it "impacts" the airplane. Stick you hand out the window of your car. The force you feel is impact pressure.
IAS is only displayed on the airspeed indicator as MPH or Knots due to convention as it really isn't a direct measure of distance traveled over time except in one condition and that's at sea level on a standard day with no wind. You could actually change the numbers on the IAS dial to A, B, C, D, etc. and learn to use it just fine, it's only by long standing convention that it's numbered as it is.
What's important about IAS is that part about it being a measure of impact pressure. Impact pressure is the physical force against your airframe due to speed and it's what lets you generate lift. The higher you are in altitude, the thinner the air is and the faster you have to fly to get the same amount of pressure against your wings to generate the same amount of lift; however, since IAS directly measures this pressure, it's the most useful indicator of how your airplane will fly. Say you're flying at 200mph IAS on the deck and your airplane can pull 4G before stalling. If you're flying at 20,000 feet at 200mph IAS you can still pull 4G before stalling but your actual speed is much higher.
Calibrated Airspeed is just your IAS after you apply corrections for your aircraft's system inaccuracy which is not an issue in AH.
Unlike IAS, TAS is a true measure of distance over time (i.e., speed); however, it is a measurement of the distance traveled through the airmass, not the ground. You arrive at TAS by taking IAS and adjusting it for altitude which is done in an aircraft manually or automatically by an Air Data Computer. In AH, this is done for you by the program. Now that you've extablished your actual speed through the airmass, you can add or subtract the speed of the airmass (wind) to arrive at your ground speed. If you're flying 400mph TAS and you have a 100mph headwind then your actual ground speed will be 300mph. In modern aircraft your ground speed is measured directly by an Inertial Navigation System or GPS. These systems can tell you the actual wind direction and speed by the difference between your ground speed/course and TAS/heading.
In AH main arenas there is no wind so your TAS will equal your Ground Speed as long as you're flying level (if you're diving straight down at 400mph TAS your ground speed would equal zero) Use your TAS as a reference point for level bombing and to determine how long it will take you to get from point A to point B. Use your IAS for everything else.