A “nulled” piece of software is a hacked and modified version of that software. It is made suitable for use without a license. Using it is illegal and there are some big risks using such software.
You are looking to safe a few bucks, at great cost
You are looking for ways to run your website the cheapest way possible. Often that results in cheap hosting and using free software and plugins. Some even go as far as using nulled or hacked versions of paid plugins.
As with lots of paid plugins, there is a nulled version of AdRotate for WordPress floating around. This, “fortunately”, is an older version of AdRotate Pro. Therefor it lacks newer features and has some bugs that have been fixed since.