A patent was a document making it possible to pay royalties to the owners of technological designs and industrial techniques.
Following Harry Mudd's departure from Rigel XII, he, in his own words "organized a technical information service bringing modern industrial techniques to backward planets, making available certain valuable patents to struggling young civilizations throughout the galaxy." Specifically, he sold to the Denebians all the rights to a Vulcan fuel synthesizer. Describing himself as a "defender of the free-enterprise system", Mudd opted not to pay royalties as a matter of principle, claiming that "knowledge... should be free to all."
Following the capture of Mudd and the USS Enterprise by the androids of the planet Mudd, Mudd jokingly told Spock that he "may be a wonderful science officer, but [he] couldn't sell fake patents to [his own] mother." Spock, not understanding the logic in Mudd's statement, replied, "I fail to understand why I should care to induce my mother to purchase falsified patents." (TOS: "I, Mudd")