The McGurk Effect is an illusion that illustrates how discrepancies in perceived stimuli can trick and confuse our brains, also showing a certain hierarchy when it comes to the "power" that each sense holds in relation to the other ones. As it turns out - they are not equal, and a human brain basically has its own "trustworthiness score".
Throughout the book a certain theme emerges: whether it comes to external input interpretation, internal cognition or memory use, our brains like things that make sense in the presented context (think: chess masters being able to remember the location of pieces from a real game, but not a random one, and sommeliers being able to remember pairings that actually go together way better than random ones), and the McGurk effect is no different.
In this illusion, a participant hears one syllable, but sees a different one (through lip reading). As it turns out, the visual input almost always seems to override the auditory one! If we see someone say the syllable va, we will hear that syllable, even though in reality a different one is uttered (ba for example). This shows that in a critical situation where the brain has to make sense from conflicting inputs, it will trust the vision more than the hearing for example, rather than creating some sort of an equal blend.
What is even more interesting, the effect seems to be universal and durable. As reported in our textbook, it worked in every tested language and still tricks researchers that spent a lion share of their careers trying to understand it. It is very interesting that a relatively simple at a first sight trick carries such an astounding amount of informative value. It also helps to put in perspective how little we really know so far, and how much else there is still to discover in the worlds of perception and neuroscience at large as well, with new studies on the subject like https://www.nature.com/articles/s41598-018-30798-8 looking at the phenomenon from different angles.