What the Heck are Stiffener Plates and What do They Have to do with WTC7?

I've been reading all the hubbub about how now that NIST has admitted that there were stiffener plates on the columns of WTC7, that their explanation for the collapse of the building no longer holds water. I had no idea what stiffener plates were or why they were relevant. So I finally googled images of stiffener plates.

Oh.  Now I get it.