Something that's inside is located near the center or in the interior of a space. The inside of a perfectly baked potato should be fluffy, buttery, warm, and delicious.
You can scrape inside a jar with a knife to get the last of the jam, or stay inside the house all day when it's very hot outside. Inside can serve as an adjective, noun, or adverb, depending how you use it, but it always refers to the inner, most privileged, or deepest part of something. Colloquially, your insides are your digestive system or stomach, and when someone's inside after stealing a car, they're in prison.