Via the BBC: The McGurk effect is a compelling demonstration of how we all use visual speech information. The effect shows that we can’t help but integrate visual speech into what we ‘hear’.
Or, further explained by Wikipedia:
The McGurk effect is a perceptual phenomenon which demonstrates an interaction between hearing and vision in speech perception. It suggests that speech perception is multimodal, that is, that it involves information from more than one sensory modality. The McGurk effect is sometimes called the McGurk-MacDonald effect.