"Prebuilt" is a general term for anything made at a factory or workshop, as opposed to homebuilt. While used for any number of things, prebuilt is most often seen in reference to computers. A "prebuilt" computer would be something like a Gateway or Dell, or one assembled at a regional manufacturer. Some people react with scorn to prebuilt computers, as they cost more than homebuilts and offer less performance. However, since your average computer user doesn't even know what a PCI slot is, prebuilts remain quite popular. The added cost of a prebuilt is simply the company covering their labor and making a profit. While I personally prefer to build my own computer, as it offers more customizability and a lower price, many people, even those that know how to build their own computer (it doesn't really require a whole lot of technical expertise, see how to build your own computer), like having the warranty and security of a prebuilt. In the end run, it's very much a matter of preference.
Of course, none of this really addresses other things that can be "prebuilt." Many consumer items can be made by an individual if they know how to do so and where to get the materials/parts necessary for building it. Furniture, watercooling devices, some appliances (no, really-- you can build your own toaster), cables (lots of people like to make their own CAT5/CAT5e/CAT6 cables), and so on. You might not hear a General Electric toaster oven being referred to as "prebuilt" (it's common to refer to the homemade items as homebuilt and not call the factory-made items prebuilt), but many items that can be made both in a factory and by an individual with moderate effort are referred to as prebuilt or homebuilt. (Zip guns don't count.)