markht has explained better than I did. These are the same stop conditions that every renderer has, max. samples, max. time or percentage convergence, whichever is reached first.

The 'Render Quality' is probably some complex mathematical thing which I don't worry about. In practice seeing the difference between a setting of 1 and 10 is going to be difficult most of the time. If you zoom in you might notice clearer reflections or sharper caustics but the extra processing will impact render times.

The setting I regularly change is the Gaussian Blur which I find a bit high at the default 1.5.

regards
prixat