Surface smoothness refers to the level of surfacing to which lumber is manufactured. There are three general levels of surfaced lumber: surfaced one side (S1S), surfaced two sides (S2S), and surfaced four sides (S4S). A combination of sides and edges can be surfaced, but this is not as common.
Rough lumber is sawn, trimmed, and edged. Due to the coarse tooth saw blades used for mass manufacturing, rough lumber can be characterized by striations or saw lines. To add smoothness and uniformity, rough lumber can be passed through a planning machine after sawing. Enter surfaced lumber.
There are pros and cons for rough and surfaced lumber. A major con of rough lumber is it requires the end user to take the time to plane and flatten it. Why risk removing more valuable lumber than you need to if you don’t have the right equipment? Another con is it is heavier and costs significantly more to ship per board foot (learn how to calculate a board foot here).
The pros of surfaced lumber include uniform thickness, ease of identifying color and grain, and it is much more efficient for the woodworker who does not have the necessary surfacing tools.