Firmware keys are used by jailbreak developers to access parts of an IPSW file allowing them to do research, or modify these parts (in a tool such as sn0wbreeze).
I learnt about these when I was writing iKeyHelper - a tool which at the time (mostly) automated the finding of these keys, using the greenpois0n payload. For those curious about how this works, iKeyHelper is open source and available to download from my GitHub. There is also a very informative article on TheiPhoneWiki.
However, iKeyHelper became redundant in February 2013, after @iNeal, @iH8sn0w and I created a set of tools which completely automate the process. This tool is now integrated into the IPSW Downloads Updater and automatically creates the page(s) that you have been visiting on this site.
Will this allow me to downgrade my device to x firmware?
Why aren't there any keys for newer devices?
Why did you make this? Surely we can just use TheiPhoneWiki?