This is for Windows.
When you connect a HDMI from you laptop to a TV you may get a picture on your TV as well on your laptop screen, you do not want to have two screen ON, one on your laptop and the other on your TV while your playing games. You can tell your laptop I only want it on my TV screen only, and this the way to do it.
Go to the Control Panel on your PC click on Appearance And Personalization and then click on Display, and on the left side of the box you will see Project to a Second screen click on that, and then a bar comes up on the right side of the screen, click on Second Screen Only and then you will only have your TV screen on only and not your laptop screen as well