Okay, so, Pandora's atmosphere is poisonous to humans, and yet there's a lot of life on the planet including a race of humanoids called the Na'vi which can obviously breathe the air and not be poisoned. Avatars are like these bodies that are a mix of human DNA and Na'vi whatever-they-have-instead-of-DNA (Way more Na'vi than human, so the Avatars look like Na'vi with a couple differences). They're made specifically for one person to use and they're insanely expensive so there's only about 20 of them, and then you get into a link unit which transfers your mind into the empty Avatar body and then you can walk around in that body like it's your own.