This is a common expression for two people that know each other and they don't meet for some time. This is not an expression for new people for us. In English, we usually say 'how are you'. Apa kabar is usually used for asking health and condition of another speaker and the other person will say ,'Baik' or 'Kabar baik'.
Bahasa Indonesia is sometimes mixed with other language for example Arabic, because it is related to the Indonesian' religion. So it is common that people in Indonesia will respond 'Alhamdulillah saya baik-baik saja' which means 'thank God I'm very well'.
Instead of 'apa kabar' we can say 'bagaimana kabarmu' and we can give the same respons as above.
If we want to know about other person condition we can say 'bagaimana kabar Susan' for example or 'bagaimana kabar keluargamu' ,you know keluarga is family.
The answer can be 'mereka baik-baik saja or they're just fine.
So, apa kabarmu dan keluargamu ?