A prefab is a reusable GameObject asset stored in the Project window. Coins, enemies, projectiles and particle effects are often prefabs because the same object needs to appear many times. Code can create scene copies of a prefab using Instantiate. (course lab series, see source-csharp-unity-labs)
The important distinction is asset versus instance: the prefab is the template, while the object in the scene is one copy of that template.
Related: unity-prefabs, unity-prefabs-scripting, prefab-vs-scene-instance, instantiate