calibration just means we are all comparing apples with apples. This is because without that your grinder could be grinding espresso at 1 or 50 depending on where you set the black ring...which is like the rotating bezel on a divers watch. So mentioning no names, but there have been people who adjusted the black clicky ring (which is just an indicator) with the expectation the grind was going to change. This resulted in some rather strange posts of people grinding really fine at 2 very different settings (again mentioning no names).
DFor anyone very new to grinders it's the silver funnel that needs to be gripped and adjusted whilst avoiding the black clicky ring (which is only used to indicate the zero point at calibration)..this might help anyone having trouble. If the top burr is moving freely (up and down) and you can turn the silver funnel and the springs are in place, then it's a physical impossibility for the grind level not to adjust.
On a properly adjusted grinder, depending on bean varietal, age dose, pressure profile and basket, then the normal range for espresso is from 15 to 24. Most people on standard machines and baskets tend to fall around the 18-21 mark.