Definition
A red herring is an irrelevant point introduced into a debate to pull attention away from the actual issue under discussion. Unlike a straw man, it does not misrepresent the opponent's argument; it simply changes the subject to something easier or more comfortable to argue about, hoping nobody notices the original point was never answered.
Example
An opponent asks why a proposed policy would cost more than the current system. A debater responds by spending their remaining time explaining how much worse an unrelated, older policy was. That may be true, but it does not answer the cost question that was actually asked.
Common mistakes
The hardest version of this to catch is a red herring dressed up as relevant, where the new topic is thematically connected to the debate but still does not answer the specific question on the table. A useful habit is to state, out loud, the exact question your opponent needs to answer, and check at the end of their speech whether they actually answered it. If they did not, naming the dodge directly is often more effective than following them onto the new topic.