You can write in Korean alphabet as "안녕"
The word was originally from Chinese vocabulary "安寧"
As Lyra said at her answer, it means "Hello" (between friends and informal way)
There are another expressions to say "Hello" according to relation between a speaker and a listener.
안녕하세요. (formal and respectful form of 안녕, also you can use in public speech to say hi to audiences like 안녕하세요, 여러분)
어떻게 지내셨어요? (more formal) , 어떻게 지냈어요? (formal) 어떻게 지냈어? (between friends and informal)
How is it going? / How are you?
오랜만에 뵙습니다 (more formal), 오랜만에 보네요 (formal) 오랜 만이야 (between friends and informal)
Long time no see.
Sometimes Koreans say 점심/밥 먹었어요? or 식사하셨어요? (Did you eat your lunch?)
It could be asked as a real question whether you ate meal or not,
but it is usually used as just say hi.
Thus if you think that is not question but greeting, you don't need to say actually whether you ate or not.
This fun youtube video will be helpful to understand this situation.
(by youtube user expatkerri )