This is similar to "How are you?" But "How have you been?" is used when you haven't seen someone for a longer time, like several months or longer.

When someone asks, "How have you been?" you should answer with more than just a short word like "Good" or "OK". You should respond by giving news about how your life has been lately. For example, you can answer:

Pretty good. Work's been busy. I've been working non-stop lately.

"How've" is short for "how have". It's pronounced this way in casual conversation.