In 1807, a journalist by the name of William Cobbett (1763 - 1835) described how, as a young boy, he would drag a particularly smelly fish, a red herring, across the floor in order to distract dogs chasing a hare. This practice, he said, was how hunters would train their own dogs to avoid distractions. In the modern day, Cobbett's story is disputed, and it is doubted that hunters ever used such a technique. However, with that article, he popularised the term "red herring", using it as a criticism of corrupt politicians and journalists.
In the modern day, the red herring fallacy remains frustratingly common. It is a dishonest strategy in which attention is diverted from the relevant topic to a new one, usually because the one making the fallacy is not comfortable with the current topic.
As an example, imagine a discussion between a Muslim and a Christian. The Christian brings up the Islamic dilemma; the problem that the Qur'an affirms that the Bible is the incorruptible revelation of Allah. In this case, if the Bible is true, then the Qur'an is false because it contradicts the Bible. If the Bible is not true, the Qur'an is false, because it claims the Bible is true. Therefore, whether the Bible is true or false, the Qur'an is false. The Muslim, not comfortable with this dilemma, attempts to divert attention away from the Islamic dilemma by criticising the Christian doctrine of the Trinity.
Obviously, the doctrine of the Trinity is not relevant to the Islamic dilemma. One could argue that a successful refutation of the Trinity would render the Bible false, therefore under the Islamic dilemma, the Qur'an is false, but of course that's not where the Muslim is going with it. They're bringing up the Trinity because they don't like the Islamic dilemma.
The best response to the red herring fallacy is to simply expose it. Point out that the topic has been changed, and either get back to the original topic, or simply disengage. From a tragic wealth of experience, I can tell you that bouncing from one topic to another until you're worn out is not fun. The alternative, however, is to get your opponent to concede. There is far more shame in pretending to know what you're talking about, and trying to give that impression by swapping topics, than in simply admitting "I don't know", and asking "can we talk about this instead?" Indeed, the latter is wisdom.