This refers to a premium, paid software that has been tampered with to remove its licensing system or "phone home" verification. Essentially, it is pirated software distributed through unofficial channels. The Anatomy of a Nulled Banner Exchange Script

A banner exchange script is server-side software that automates reciprocal banner advertising between websites. Participating publishers display other members’ banner ads and, in return, earn impressions or placements for their own banners across the network. Operators may use such scripts to grow referral traffic, increase ad inventory, and build niche ad networks.