Patent medicines are medicines that are sold and advertised as being cures, as being something that will solve a medical problem, but typically do not.
When we're talking about physical things, then we use the word " patent." Patent medicine, however, isn't patented medicine, so it's something of what we would call a misnomer.
In other words, some of the basic principles and ideas of advertising, of marketing, came from people working in the patent medicine industry, in the patent medicine business.