Welcome! This site is imho the best resource for the PPC6700 on the internet. Right now much of our direction is towards custom roms. These are upgrades to your operating system. Kind of like Windows 95 to Windows 98. Except many (freeware) programs are included right in the install!
You may hear things about AKU 3.3, 3.5, 2.2 and so on.
AKU's are just revisions to Windows Mobile 5, which is what our PPC-6700's use.
Just read the sticky's and get a general feel for the site layout, and do a search if you are having problems first instead of starting a thread. Keep in mind we are a community, and we are all here of our own free will, so please keep that in mind when asking for help.
To answer your question:
A "registry" is something that is common to all windows based machines.
http://en.wikipedia.org/wiki/Windows_registry
You can edit things in the registry to fix or change functionality of your phone.
I would suggest holding off on reg edits for a while, and stick to patches that people make. Also, feel free to PM me if you are unsure of something that you think may have already been addressed... I'm on all the time XD.