A laptop computer cannot be used as a second monitor for a PC in the traditional sense. Most laptops have video out ports to enable attaching a secondary monitor to them but lack the necessary video input for a laptop to be used as a second screen.
The solution for those who wish to use a laptop as a second monitor is to use third-party software. These types of programs allow the user to share one computer's screen with another PC, although the sharing is strictly simulated. While the screen can be shared, both host and client computer must run the software to enable it.