Hi!
안녕하세요(an-nyeong-ha-se-yo) = Hello. / Hi. / How are you?
안녕 + 하세요 = 안녕하세요.
an-nyeong + ha-se-yo = an-nyeong-ha-se-yo
안녕 = well-being, peace, health
하세요 = you do, do you?
안녕하세요(an-nyeong-ha-se-yo) is the most common way of greeting someone in Korean and it is in formal or polite way.
Or 안녕(an-nyeong ) will do also.
hope it helps..=)