A "strawman" is a kind of logical fallacy. It's when somebody ignores the actual argument someone is making and substitutes a fake or misrepresented argument instead. This kind of argument is very popular in politics. For instance: "Mr. Smith believes that all cars should be banned, and no one should be allowed to use electricity so that we can save the environment!" This would be a "strawman" argument because the person speaking is using an exaggeration of his opponent's position.

It means an issue that someone has raised, but that the speaker thinks is bogus or a diversion from the main topic.