What are the differences between quality levels?

The biggest difference is the visibility of the layer lines.

No matter what quality level you choose, you will see visible layer lines in FDM. At a higher quality setting, you may see them less and they may be easier to smooth out in post processing steps after printing. However, printing at a higher quality setting means it takes longer to print.

In SLA printing, layer lines are typically less visible than in FDM. Printing at a higher quality setting in SLA means that your parts will appear with virtually no layer lines. As is the case with FDM, SLA also takes longer to print in a higher quality setting.