What is an active S-Box? I don't understand what an active s-box is (in reference of design criteria 8 of the DES). 

Because according to DES, all 8 s-boxes are used every round. But this is not what active means.

So my question: What is an active S-Box, and what does it mean when an s-box is active (or when two or three s-boxes are active)?
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).