Aspect ratio is length x width. Old tube TVs were 4:3 aspect ratio, new widescreen displays are 16:9
Display scaling is when you try to stretch a 4:3 picture/signal to fit a 16:9 display.
Scaling can leave the picture in its original size and you see black bars around it, or you can stretch (scale) the picture to fit in the larger display.
The subject has much more than what I just listed, but for an overview - that should be good.
Hope that helps