Salmon skin, I’m guessing most people have seen this.

In a word, salmon skin is the result of vibrations. Your nozzle isn’t exactly where the printer thinks it is while it is extruding, which causes variations in the surface texture of the finished part.

To improve your surface finish, go through the components of your printer and make sure they are tight. Like many things, some parts of your printer loosen up over time. It’s a good practice to make sure everything is tight periodically anyway. Make sure all of the bolts and nuts are tight, make sure that your roller bearings are smooth. Make sure that your printhead doesn’t have any slop in it if you try to move it with your hand, etc.

Regular maintenance checks should get rid of most of the effects of salmon skin.