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Is a pretty broad term that will likely get whittled down over the next few decades.
But classically, the inversion or scrambling of words, letters and numbers in your mind's eye and expression is what most people think of as dyslexia. This really doesn't indicate a decrease of intelligence or problem solving; it's more along the lines of color blindness, or another form of sensory obstruction.
What happens is the interpretive/expressive part of the brain gets what amount to little spasms, or seizures, or cramps when it's doing its job. What happens when your muscle cramps up during a weight lift? Naturally, it impacts your performance, but it doesn't mean your strength (cognitive ability/brain power) is in any way lacking.