[ELI5] What exactly is dyslexia?
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Dyslexia is a cognitive disorder that make reading difficult.
There are generally two main types of dyslexia: phonological dyslexia and lexical dyslexia.
These types affect different cognitive paths in the brain.
The phonological path is reading a word letter by letter or syllable by syllable. This is the way children first learn to read and this is the way adults read words that they are not familiar with, proper nouns or invented words or series of letters (logatomes). A person whose phonological path is affected by dyslexia will have troube reading this way, in particular with the seriation (putting sounds in order) of the letters and sounds.
The lexical path is the expert way of reading. When children become more adept at reading, they no longer read syllable by syllable, but rather sort of photograph the word they see and match it with an internal visual representation of the word that they recognize. This representation is linked to others that contain the phonological, semantic, metalinguistic informations. A person which lexical path is affected by dyslexia will have trouble matching the word to a representation and that can result in issues where words are not well identified or even replaced by logatomes (coffee can become goppee for instance).
This is all closely related to working memory and attention, so the intensity of the disorder might depend on things like concentration or being tired. People that are not dyslexic might exhibit symptoms when very tired or otherwise cognitively impaired by substances that affect high cognitive functions.
I will add that this information comes from cognitive psychology courses taught by speech therapists about 10 years ago and repeated here off the top of my head.
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