Coucou !
"Hi there" or "Hello". It's used, in colloquial language, when the greeting is a surprise or meant to be a surprise eg when a friend who turns up suddenly and says "coucou" it means "you were not expecting me, eh !" or an email arrives from someone you haven't heard from for a while. It's also used in children's play with the sense "peek-a-boo".
It sounds like and seems to be linked to the way a cuckoo pops unexpectely out of a cuckoo clock.
When you arrive at a friend's house you can say "Coucou, c'est moi ! - Hi, it's me!" as you enter or if you want to check if anyone is home you would say "Coucou, il y a quelqu'un ? - Hello, is anyone there?"
Au sens strict lorsque c'est utilisé comme salutation c'est surtout quand la salutation doit être une surprise ou doit créer une surprise. Un peu comme la sortie du coucou de l'horloge surprend. C'est le cri des enfants qui jouent à cache-cache.