Plain English Meaning
When the wind is gusty, you fly the approach a little faster than normal so a sudden drop in wind speed doesn't leave the airplane too slow to fly safely.
Full Definition
The additional airspeed margin a pilot adds to the normal approach speed to compensate for gusty wind conditions during landing. A common technique is to add half the gust value (the difference between the steady wind speed and the peak gust) to the approach speed, so the airplane retains adequate control authority and stall margin if the wind suddenly drops.