metldr key?

I just got my first PS3 so I decided to introduce myself to PS3 homebrew. I have some questions though.
What can I do with the metldr root key that geohot released?
Can I use it to sign homebrew ps3 apps and run them natively without CFW?
Or is it used to sign the CFW itself???
Whats this? app-priv-rev1: 00 3d e8 01 67 d2 f0 e9 d3 0f 21 45 14 4a 55 8d 11 74 f5 41 0c
Finally if I can't use the root key to sign apps; What key do I use? And how can I obtain it?

The root key was never released. Geohot only released a key to sign homebrew for 3.50 firmware and remove the sig check. You need CFW to run homebrew.