A more thorough explanation:

Fox's Libel Act is a law that was passed in 1792. It changed the way libel cases were handled in court.

Before the law was passed, if someone was accused of writing something that was considered libelous (meaning it could harm someone's reputation), the jury had to find them guilty if they believed the person had actually written it. They didn't have the power to decide whether the statement was actually libelous or not.

But Fox's Libel Act changed that. It gave the jury the power to decide whether the statement was actually libelous or not. This meant that even if they believed the person had written it, they could still find them not guilty if they didn't think it was harmful enough to be considered libelous.

For example, let's say someone wrote an article about a politician that was critical of their policies. The politician could sue them for libel, claiming that the article was harmful to their reputation. Before Fox's Libel Act, the jury would have had to find the writer guilty if they believed they had actually written the article. But after the law was passed, the jury could decide whether the article was actually harmful enough to be considered libelous.