Monitor minimizes in middle of video stream

Hi, when I try to stream movies via hdmi cable from my computer to my tv, I can watch about 15 minutes, and then the picture minimizes, and I have to get up and maximize it again.
I get a message when the video stream starts saying in order to enjoy uninterrupted viewing, I should disable my computer screen. Do you have any idea what this is talking about and how I can do it? Thanks!!

have you turned off the screensaver?

How do you do this?

right click on the desktop, then choose personalize, then at the bottom of the window that opens you can select your screensaver, choose none!