There are several settings that define the surface quality of the print; the most important ones are described here.

Layer height

The layer height is probably the most often changed setting. By default the layer height is set to 0.1 mm, but you can of course use thinner layers to increase the quality or use thicker layers for faster prints.

Shell thickness

Furthermore the shell thickness is a setting that is shown by default in the Advanced mode. This setting defines the thickness of the wall of the print. Based on the set shell thickness Cura will calculate how much lines the wall should exist of. A shell thickness of 0.8 mm means that it will print 2 lines (2 times the nozzle diameter of 0.4 mm); this is strong enough for most prints.

Bottom/top thickness

The bottom/top thickness defines the amount of solid layers of the bottom and top of the print. By default this is set to 0.8 mm, but it might be necessary to increase this in some cases. For example if you have an object with a big flat top surface or want to print with a low fill density more layers may be required to completely close the top of the print. Using a too thin top layer (in combination with little infill) could lead to what is called "pillowing" in such a case (see below picture).

Pillowing on the top surface of a print

Bottom/top pattern

This setting allows you to change the pattern of the bottom and top of your print. By default it will just print lines on the bottom and top layer, but it can aesthetically be interesting to have a different pattern. Instead of using lines you can also choose for a concentric bottom/top pattern.

The bottom/top can be printed as lines or concentric