The outside of something is the part that's visible to the world. A neighbor who's used to the tidy outside of your house might be shocked by the mess inside.
Your perfect chocolate chip cookies might be crunchy on the outside, but soft inside, and when your friends eat them, they'll look as happy on the outside as they feel inside. Outside is also an adjective, for talking about the outside temperature or your outside lights that automatically turn on at dusk. If you're on the outside of some group, you don't feel like you belong, and if someone talks about "an outside chance," they mean a very unlikely possibility.