NULLED mean “Already activated with license key”. The original versions of some products do not work without activating the license key. In order for the product to work fully without activation, it is necessary to make minor changes to the part of the php/js code to bypass the license protection. The product in which such changes are made is called NULLED.
When you purchase a NULLED version of the product, you also get its original version (files without changes), so that if necessary, you can make sure that the changes made to the product concern only the protection bypass do not affect the functionality of the product and there are no malicious code or ads in the files.