Usually, it's just an informal way of saying "hello." However, in some contexts or with a certain voice inflection it could be considered flirtation.

Hi! It basically just means hello. It's informal and a little endearing, used especially if you see someone you haven't seen for a long time. I guess, in that sense, the "there" sort of emphasizes the presence of the person who wasn't there before. But you can think of it just as an informal greeting.

It´s kind of short for saying hello you standing (over) there, which derived from archaic English