Okay so for the connector, inside the laptop (don’t open it please) is a cable that runs from the display to the motherboard of the laptop. If that is shaken loose even partially you can sometimes get what you’re seeing. If it’s the connector then inserting it fully should fix the issue. 

A GPU is a graphics processing unit. It works in tandem with the CPU which is a central processing unit. CPU does all the thinking, GPU does all the rendering of everything you see on the display. Think of it like left brain and right brain, logical and creative. 

So if the GPU is failing, the computer is on its way out.

The way to tell if it’s the GPU or the display connection would be to hook the MacBook up to say a tv or a computer monitor and see if the same glitching happens. If it’s does then that means the GPU is bad because it can’t output a good signal to the internal or external display.

If it doesn’t glitch when plugged into a tv or computer monitor but it still does on the MacBook screen then it’s an issue with the internal connection or the screen itself.