True Airspeed (TAS) is the aircraft’s actual speed through the air mass. It is IAS corrected for air density (mainly altitude and temperature). As you climb and the air gets thinner, IAS decreases for the same aerodynamic “feel,” but TAS increases for the same IAS. That is why aircraft cruise faster (in true speed) at altitude even when the indicated speed looks modest.
In DCS World, TAS matters for navigation timing, fuel planning, intercept geometry, and weapon/arrival timing. Many jets display TAS on an avionics page, the HUD, or the data sublevel, and you will often use TAS when thinking about “how fast am I really moving through the air,” especially in high-altitude cruise.