A prefab is a type of asset you’ll store in your project. They act something like a blueprint for objects. You set up a prefab for some kind of object in your game and then add instances of the prefab to your game, in any of your scenes. This connection to the prefab will remain across all the instances, and you can then change the prefab itself to automatically change every instance you’ve placed.
For example, you might make a prefab out of a certain enemy type. You place that kind of enemy throughout your game levels (scenes) possibly hundreds of times. Somewhere down the line, you ...