Something extreme is far out, either in terms of distance or intensity. Extreme sports are wild or dangerous, and an extreme height is very high.
When you talk about extreme things, you're talking about things that are very, very...something. An extreme point is the farthest distance from a place. Extreme eating is a type of contest — like hot dog-eating — in which people eat a freakish amount of food to compete with each other. An extreme biker is biking down dangerously steep mountain sides. Anything extreme is pushing the limits and taking things about as far as they can go.