Unity's Prefab system allows you to create, configure, and store a GameObject complete with all its components, property values, and child GameObjects. See in Glossary as a reusable Asset. The Prefab Asset acts as a template from which you can create new Prefab instances in the Scene. See in Glossary.

What are prefabs for?

Prefabs are a special type of component that allows fully configured GameObjects to be saved in the Project for reuse. These assets can then be shared between scenes, or even other projects without having to be configured again. This is quite useful for objects that will be used many times, such as platforms.

Where are Unity prefabs?

With Unity opened you can go up to Assets>Create>Prefab. You'll notice that a new icon gets placed within your assets window. You can name it whatever you like.