As said before, the High Concept is the basic description of the character. Luke Skywalker is “The Son of Skywalker”. Robin Hood is “The Outlaw Noble”. Indiana Jones is the “Seat-Of-His-Pants Archaeologist”. While there’s obviously more to them than just that, it’s a good quick summary of the high point.
There’s a lot of variety available for characters. While you could easily go with an Atlantean solider or scientist, there’s no reason you can’t go with something a bit wilder, if you want: one of the other alien races; one of Atlantis’ genetic experiments; the sky is pretty much the limit. I can’t guarantee that every option is viable, but there’s a lot of room to work with, and for the most part, you can play what you’d like (within the limits of the character power level.) (If you would like a list of the type of people you’d likely find in Atlantis, look here.)
Something you should keep in mind, however, is the setting: while gameplay starts in Atlantis, spoiler warning, the city doesn’t last forever. PCs should be able to survive in both high- and low-tech environments. There’s going to be a reasonable amount of conflict in the game – how they get dealt with is going to depend on the characters. (ie, if there’s a bunch of combat-oriented folks, I expect a reasonable amount of combat; if everyone’s a diplomat, there’ll likely be some diplomatic options available too.)