Unusual word combinations, basically. They’re word combinations that are rare in the training data, i.e. rare in normal language, but used more frequently by AI.

They’re difficult for humans because if a sequence of words hasn’t previously been seen, you have to expend more effort to make sense of it.

Something like “customer onboarding workflow automation" might qualify - it makes sense, but it’s not a commonly used phrase. AIs use a lot of these jargony sequences.