Thanks for the info!
I'll check out the other links.
Ah, I see what it does now:
This only applies when the viewport is being stretched (i.e. Fullscreen mode is not Off). High quality mode renders at the full resolution of the displayed size. Low quality mode first renders at the project viewport size, and then simply stretches the result to fill the screen. Low quality mode often improves performance on low-end systems and is often suitable for retro-style pixellated games with Point sampling. However note that text, downscaled sprites and effects will appear with better detail in high quality mode."
Probably the reason I'm not seeing any perceptible loss in quality is because it's not being stretched a great deal.