If I run my finger over the top of my MacBook one of the words that comes to mind to describe it is smooth however that isn’t the only meaning that smooth has. Yes a surface can be smooth or free from unevenness of surface however it can also has half a dozen other meanings.
It can refer to how easy someone made something seem. For example I am entirely uncoordinated and I am not the best at basketball, however if you get LeBron James in the WoodPEC he would make it seem like second nature. “That was a smooth shot”.
It can refer to easing tensions with someone. For example, maybe me and my room mate got into an argument last week over me not keeping the room neat. Afterwards I could clean the room to smooth things over.
If I were running a race however I had run away from most of the competition and I had only 50 meters left, the commentator could say he has a smooth ride into the finish, suggesting how I do not have any competition or obstacles in my way from winning.
The saying “Through the rough and the smooth” means that you will stick with someone through both the hard parts, rough as well as the easy parts, smooth.
All of these examples go to show how smooth has a multitude of meaning not just the physical one of a smooth surface, but a “mythical” meaning in culture. A meaning that has been derived from its original tactile meaning to all aspects of life. If we are ever unaware of the meaning of smooth in context we can always work backwards from the tactile meaning and use context cues to figure out what it means in a specific example. For example prior to my research into smooth I had never heard to saying “Through the rough and the smooth”, I had heard a smilier saying, “Through the thick and the thin” however from my familiarity with what smooth and other sayings means I was able to figure out what smooth mean in that specific context.