High Definition Monitor Why Cant My Monitor Display High Definition Fullscreen?

Why cant my monitor display high definition fullscreen? - high definition monitor

I have recently bought x233h Acer monitor and when I try, games or video format 1920 play x 1080 screen that gives me a black armband on the screen, which defeats the purpose of why I purchased. W Ould like to know whether anyone else this problem and it is possible to work around. Thank you.