This is a pun on the word "hi". The word "high" has been replaced by the similar-sounding word "hi" because "hi" is a greeting that can be conveyed by waving at someone and this joke is about someone who waves at people instinctively. It is natural because he didn't intend to actually greet anyone, he waved without thinking about it.
A boy who instinctively waves at people has a natural hi .