I don't know about it plain English, but I will give it a try.
Windspeed is defined by the time between debounced "ticks" coming from the anemometer. So, it is time between the interrupts (debounced) from the Windspeed. Faster the anemometer turns, the shorter the time interval. The debouncing limits the wind speed number to somewhere around 120MPH.
Wind Gust is defined by the shortest time between anemometer "ticks" during your sampling time. Remember that both of these wind measurements are defined by the speed of the interrupts coming from the anemometer into your computer. So, the most important part of the code is that interrupt service routine, shown below: