It's often used to describe something that is located outside of a particular structure or system.
Here are some examples:
* Exoskeleton: The hard outer covering of some animals, like insects.
* Exosphere: The outermost layer of the Earth's atmosphere.
* Exothermic: A chemical reaction that releases heat to the surroundings.
* Exoplanet: A planet orbiting a star outside our solar system.
So, when you see "exo-" in a word, it usually suggests something located outside of something else.