Video quality looks different on different computers.
Youtube video quality is related to the speed of your internet connection and your own computer’s resources.
To the right of the red line that moves across the youtube player screen when you watch a video you’ll see an arrow and some resolution numbers, usually ranging from 380p to 1080 these days.
You can click that arrow and try to play the video at a higher resolution.
If your internet connection speed and resources are straining then the video will look blurry, and play with a jerky movment instead of a smooth play.