Parse, recall, and produce language quickly.
This is the set of skills involved in recognising words, recalling correct spelling, and producing language accurately — reading, writing, and verbal recall working together.
It's behind everything from spelling a word correctly under time pressure to finding the right word mid-conversation without losing your train of thought.
Word-finding and spelling can be affected less by language ability itself and more by attention — losing your place mid-word, or the working-memory load of holding a whole word's spelling in mind while writing it out.
Knowing exactly how to say a word out loud but blanking on how to spell it the moment you sit down to write it.