Can anyone explain to me why I see a yellow or red checkered pattern when my computer or browser is struggling to render a page? I tried to take a screenshot,

Can anyone explain to me why I see a yellow or red checkered pattern when my computer or browser is struggling to render a page? I tried to take a screenshot, but this usually happens super fast and I can't react in time. I'm on a MacBook Pro and running Chrome (although I get the checkered yellow pattern in other applications like iTunes as well).