I take it to mean that they're composed well: there's a balanced and sensible spacing/padding, the lines are a good weight, they are easy on the eye, there's a consistent look and feel between all of them etc etc.

For instance, I could mangle some SVG icons and while you could scale them to an infinite size.. you wouldn't want to use them ;)

These look great BTW!

Quality as in "a BMW is of better quality than a Peugeot" ;)