Somebody else may have a better definition but here is something basic.
A nulled script is when somebody changes the script to remove the protection implemented by the author of the script. For example, when they remove call homes or registration checks or etc. These scripts are commonly distributed by warez sites and etc shady/illegal sites. Illegal software.
You've hit the nail on the head, Carl. That's exactly what they are.
A good way of checking whether the software is nulled/illegal is this: take the script name, and google it. If you go to the script's site, and it costs money, but you're getting it for free, then it's probably nulled.
Nulled Scripts are dangerous not just illegal i remember not knowing what a nulled script was and it came with malicious embedded code got my hosting suspended and almost crashed the entire webserver i was hosted on quiet simple .... dont use them... if you cant find what you on on open source get a few of them with the features you want and make your own software of course complying with the acceptable use policies and terms of service.
one thing to remember if you dont know ask thats how you learn and if the person you ask doesnt know they'll often know someone who does if not then a simple google search can bring many tutorials. the possibilities are endless with open source.