New born infants possess an exquisite ability to learn the sounds of the speech that they hear. Speech is made up of individual sounds called phonemes and there are about 400 phonemes in the languages of the world. English uses a little over 40 of these sounds and the range of phonemes in world languages is 12 at the small end and 112 at the large end. The ability to learn these sounds greatly diminishes during the early months of life and while a 3month old child can readily learn all of the 400 phonemes, a 12 month old has lost the ability to capture many of the phonemes contained in languages the child is not being exposed to.
Newborn children display some variability in being able to learn the phonemes making up their language and children who are especially poor at this have what is called a phonological core deficit. As explained on the overview page, we can think of this deficit as involving an inability to hear sharp distinctions between the sounds of phonemes. This inability can have numerous consequences including making it more difficult to learn to speak, making it more difficult to correctly pronounce words, and making it more difficult to learn to read.
It is believed by most reading researchers that this underlying difficulty in learning the sounds of language is the root cause of dyslexia.