What is an active S-Box?
You run the algorithm with two different plaintexts (whose difference is usually small – just a few bits, everything else being equal). 

Wherever these plaintexts lead to different inputs to an S-box (in any layer/round of the algorithm), we call this S-Box “active” (since the other S-boxes produce the same result for both plaintexts, they are called “passive” and are not considered further).