One thing every person (and most animals) in the world has in common is a skeleton: we all have a connected group of bones that help shape and support our bodies.
You probably see a lot of skeletons at Halloween, because they look a little spooky. They shouldn't scare you too much, because we all have skeletons under our skin and muscles. Skeletons give our body shape, and parts of the skeleton (like the skull) protect important organs (like the brain). Sometimes, the word skeleton is used to mean the "bare bones" of other things: the skeleton of a financial plan is a basic outline of that plan. If you said someone has a "skeleton in their closet," it means they're hiding something embarrassing or incriminating.