Yardage adjustments on front of scope.

[TNRifleman]

Most likely parallax adjustment.

 

[fairchaser]

As stated it’s to adjust for parallax. Past a certain distance it should be parallax free. You might notice the scope is a little clearer when you make the proper adjustment. Also when adjusted you should be able to move your eye around while looking through the scope at a fixed target and the crosshairs shouldn’t move around the bullseye.

 

[Andy S.]

^^^What they said.

http://www.larrywillis.com/tip015.html