“Yes,” “No,” “I see,” “Got it,” “Here,” “Still here,” “Here you are,” “OK,” “What?” Hard to believe one single word can have so many different meanings. Particularly if it’s one that looks as innocent and undemanding as “hai.”
This false first impression may result from hai’s famous but dull part in the old response pairing of はい (hai, yes) and いいえ (iie, no). But just as communication involves a lot more than just answering yes/no questions, that’s only a tiny bit of what はい can really do.
Likely its most important function lies in the domain of backchanneling, 相づち (aizuchi) in Japanese. Together with its more casual peers うん (un), えー (ē) and a couple of others, it occurs in high frequency in everyday conversations. “Go on, I’m (still) listening” is what it says to the current speaker, if not in so many words.