The term "red herring" gets thrown around fairly often, especially when discussing mystery genre stories, or other stories with such elements. Based on the context that you've heard the term used, it should be pretty obvious what the term means, but what happens when the obvious answer is wrong? It would be such a shame if the author of something you were reading deliberately misled you by placing clues in the story for you and the characters to incorrectly assume are the answer to the big mystery. But, it happens, and you should really watch out for it when it does.