Not dyslexic, so I have no idea what the experience or sensation is like. But you asked how it is explained. So here is how I heard it explained once.

Consider a CPU in a computer. It can perform a wide range of tasks, literally anything if given enough time. There is a region on it called the FPU, or floating point unit. It is specialized for doing one specific task, iterated mathematic operations with one specific type of number. This section has a cache and a stack (speciallized types of memory,) so the CPU can just let it do its own thing for a while and know that the correct answer will come out automatically without any additional instruction. This feature is especially useful for figuring out what color to make a pixel on a monitor.

If the FPU was broken and output an incorrect answer, then the CPU, if it was concious and able to adapt could still calculate what color to make each pixel on the monitor. But it would have to ignore the FPU's output and perform each calculation manually, over and over again.

Similarly, the human brain has specialized areas for processing written language, spoken language, arithmetic, etc. We don't know exactly how they work hut we know they exist. So we can, for instance, parse the meaning of a sentence we read without putting any thought into how we are doing it. It just pops up automatically.

If one of these structures has a faulty output, then the brain needs to ignore the incorrect results it automatically outputs and go over the data step-by-step to process it.

Exactly what this would be like depends on which structure is faulty and how inaccurate is its output. Probably mostly incomprehensible to someone without a similar experience, though.