Imagine you have a cake. You cut this cake in half. You continue cutting the leftmost half in half in the same direction you did the original cake forever. This takes an infinite amount of time. Let's say you want to cut this cake into an infinite number of slices in 2 minutes. You take 1 minute to cut the cake in half the first time. Then you take half the time for the next cut. Continue taking half the time of the last cut until you finish. Cutting the cake in that way is a supertask. Doing an infinite amout of stuff in a finite amount of time is a supertask.