What's the difference between low- and high-quality STL files?

Models in STL are basically bodies created from triangles (facets) or polygons, so you need to configure tolerance levels that “smoothen” the surface of the bodies and make them more accurate. The smaller the tolerance level, the more accurate the surface’s curves are presented.  The deviation and angle tolerance levels alter the quality of your STL export. 

If you intend to use 3D printing at home with your filament-based printer, then you can use low-quality STL. But if you intend to build the models with CNC machines or an advanced SLA printer (such as a Form 2 printer), you should use high-quality STL.