What are all these metal cans/pods sitting on top of electronics?

This is a form of mass loading, a type of vibration control. Nothing special, just solid mass. Looks better than a phone book, one of the first I tried 30+ years ago. That's how old this is. You can try a book, dive weights https://systems.audiogon.com/systems/8367 lead shot, granite, butcher block, sand bed, literally anything with mass. Not all materials sound the same. 

Look close, you will notice I have done this with a lot of mine. 

I kid you not. They sell bags of rocks. 160. usd. My setup would have taken 1600.00 worth of "ROCKS" to load my speakers properly.

390. each x 5 bags x 2 = 10 bags of rocks for 1600.00 usd.. Nice bags though..  I like the idea, just not the 1600.00. OH They had "Earth Boxes" from 1k - 10k.. NO KIDDING.. Box of dirt.. 10K.
I'll take 5.. Keep the change!

Cabinet material makes a difference too.. My Elixirs are HDF. Go tap on the side walk.. about the same.. The monitor section, 390 lb..

Regards

I would not buy any of these items unless they have undergone cryogenic treatment and quantum cleaning. Be sure their astrological sign is aligned with yours.