Making sure your extrusion settings (or E-Steps) are correct is mandatory to have successful, clean prints. Under extrusion can lead to a print that doesn’t finish and over extrusion can lead to very poor surface quality.
Under Extruding will have prints with gaps in the top layers, they wont be watertight, and they will not be as mechanically strong as they could be. Overhangs are also very difficult to print when under extruding. However, you will not notice much of a surface quality problem (other than perhaps layer delamination).
Over extrusion will cause prints to be too large, holes to be too tight, surfaces will look overstuffed and cause layers to be bulging out. Since the printer is over feeding the filament, each layer will likely be pushing into the layer below it.
This leads to very poor surface quality prints. While this is true, overhangs are much easier to print when over extruding.
E-Steps refers to the amount of steps the motor should move forward when feeding filament, in a steps/mm measurement. In order to calibrate this, you will need to compare the amount of filament that should be fed through your extruder vs the amount that is actually being fed. This involves measuring out a certain amount of filament and then telling your printer to extrude that amount. You can then measure the difference between what should have extruded vs what was actually fed.
Using a simple calculation can help you flash your firmware with the correct e-steps.
There is a very helpful video tutorial on this you can access by watching Thomas Sanladerer’s YouTube Channel