Plain English Meaning
It's how fast the airplane is really moving through the air around it. The airspeed gauge in the cockpit reads low at altitude because the air is thinner up there, so true airspeed is the corrected, real number.
Full Definition
True airspeed is the actual speed of the airplane through the surrounding air mass, corrected for the effects of altitude and non-standard temperature on the air density. It is calibrated airspeed adjusted for the fact that the air at altitude is thinner than the air at sea level, where the airspeed indicator is calibrated to read accurately.