Basically, e-steps determine the rate at which filament is pushed through the nozzle of the printer. The default value in the Snapmaker firmware is often not quite right for individual machines, causing them to overextrude (push out too much plastic) or underextrude (push out too little). One possible cause of texturing on the upper print surface is underextrusion leaving space between the lines of plastic being extruded.
The instructions for calibrating e-steps are here: