Nulled software (such as a WordPress plugin or theme) is a premium product that has been stolen, probably modified, and available for free. Theoretically, any licensing calls to the developer’s site have been removed (i.e. nulled) from the software, but who knows. The problems posed by using nulled plugins or themes are numerous: for starters, you are stealing, and besides, the software is likely to include malware. Just say no!
For ideas on where to find legitimate free themes, read 7 Reliable Sources for Free WordPress Themes. And for free plugins, use the WordPress Plugin Repository.