Adjustment per click is, how many centimeters/millimeters will move on the target at 100 m when you make one click on the turrets for elevation or a point of impact windage on the rifle scope. Clicks are usually specified in MRAD (1 mrad is 10 cm / 100 m) or MOA (1 MOA is 2.9 cm / 100 m). Some manufacturers designate MRADs with acronym MIL.
Practically all newer rifle scopes have the possibility to adjust reticle left or right (windage) and up or down (elevation). This process is known as zeroing. The upper turret on the rifle scope is for elevation adjustment of the reticle and the side turret on the rifle scope is for windage adjustment of the reticle. Hunting rifle scopes have the mechanism of both turrets protected with caps that protect the turrets from water, damage or any other outside impacts. Turrets are in other words rotatable buttons which you can spin left or right.
Every single movement made with the turret produces a »click« sound. Usually, 1 click on European rifle scopes moves hit on a target for 1 cm at 100 m range (0.1 MRAD / MIL). On American, Japanese and Chinese scopes 1 click moves the hit on the target for ¼ MOA (minute of angle) which is 7 mm at 100 m range. On Benchrest rifle scopes, where corrections have to be very small, the clicks are either 1/8 MOA (3,5 mm / 100 m) or 0,05 MRAD (5 mm / 100 m).