A turnkey project is one that is fully operable when it is handed over to the project manager. A turnkey project can exist within a business setting or can be the actual business.
In a business setting, a turnkey project is generally produced by a team of contractors and project leaders who have experience with the type of project that is being produced. These contractors will do everything needed for the project. A project idea is often handed over to them and they are expected to produce the project up to the standards set by the company they are contracted with. They will be able to do everything from the design of the project to the minor details. This project is then handed over to the project manager, who functions as a customer or client in this situation.
Turnkey means that the project manager will only have to turn a key to have the business or project up and running. Although not every project has a literal key, it is figurative. The project manager or client will only have to do very minimal work. They can expect a turnkey project to be returned to them and fully operable when they are ready.