A patent troll is an unkind phrase used to describe a company or person that owns a patent and tries to get other people to pay a lot of money for the right to use the patent. The patent troll usually asks for an amount of money that is less than defending a lawsuit.
A patent troll is different from other patent owners for two reasons. First, the patent troll usually does not make or sell any products itself. Second, a patent troll is also not usually the inventor. The patent troll instead buys the patent from other companies, and, usually, very cheap because the other company was going out of business.
Patent trolls make a lot of people unhappy because the money paid is a lot like a tax. The term troll itself is meant to invoke the image from fairy tales of a mean monster living under a bridge that makes people pay to cross the bridge.