Everyone above me is technically right, but let me add four points of perspective so you can better understand the meaning.
First is that direct address using someone's name is extremely rude in Korean culture (not so much in Gyopo culture, like Korean-American but definitely rude in Korea) if they are older than you.
Second, in Korean culture (not so much in Gyopo culture) you can only be friends with someone exactly the same age as you. If they are one year older or younger, they cannot be your friend. You can always use the least polite level of formality with someone the same age as you, unless they are in a position of power over you (ie. the President, the military, the boss at the office, a pastor or priest).
Third, gender is extremely important in figuring out how to address someone (true for Gyopo culture too) directly without using their name.
Fourth, there are 7 levels of formality in Korean culture, and Koreans conjugate their sentences based on how formal they should be (not subject or verb agreement) and it is only okay to speak familiarly with someone at the half-word levels (ban-mal [반말]), which are the two lowest levels of Korean (해체 and 해요체).
So if someone can only be your friend if they are the same age as you, and you want to be American style friends (age doesn't matter) how do you address them?
Well, girls address older guys as (오빠) Oppa. This indicates to the older person that they want to be friends without being disrespectful.
Additionally, when girls want to specify whether or not they are interested in a guy or if he is just a friend, they specifically say that he is just an Oppa (meaning he is like a brother and just a friend to her) rather than a boyfriend or Eyin (애인).
So the literal meaning is "older brother" and it also sometimes means "friend" but its usage can be a little complex at first.
Hope that helps.
2004-2006 Korean National Speech Contest Grand Champion