Plain English Meaning
Two numbers describing the same decision point on an approach. One tells you the altitude on your altimeter at that point; the other tells you how high you are above the runway you're aiming for.
Full Definition
DA/HAT is a paired value shown on instrument approach charts for precision and approach-with-vertical-guidance procedures. DA (Decision Altitude) is the altitude, expressed in feet above mean sea level (MSL), at which the pilot must decide whether to continue the approach to landing or execute a missed approach. HAT (Height Above Touchdown) is the same point expressed in feet above the of the of the runway. numbers refer to the same physical point in space; they are simply measured from two different references.