A nulled plugin is a hacked version of a real WordPress plugin.
Nulled plugins are obtained and distributed without the permission of the company that made them. They're usually hacked and sold for a lower price, or given away in a huge bundle.
Nulled plugins are modified so you don't need a valid license key. But in addition, nulled plugins often include extra code that could infect your site with malware and prevent you from getting security updates.