A lot of people have the misconception that a sniper can easily hit a target by placing it in the center of the crosshair and pulling the trigger. We know that the bullet's trajectory is parabolic, that a handgun bullet has a discernable difference at 15 meters, and a rifle is about 70 meters, so depending on the distance, the bullet can land at any point along the vertical line of the cross, and that the sight is not a guide but a reference. To hit the target accurately, a sniper must accurately estimate the target's distance. What the hell are the "ticks" and "numbers" in sniper scopes for!
The special line of sight in the scope can be used to estimate the target distance or take the lead of the shot. Usually used by snipers. The so-called "mil" is milli-radians. A circle is equivalent to 6.283 radians and a milliradian is equivalent to 0.0573 degrees. Since it is an Angle, like the application of a indexing Angle (MOA), it can be used to indicate a specific height or width at a certain distance. Tan (0.001 rad) * 1000 = 1.0000003333, so one milliradian at 1000 meters equals one meter height/width.
Between the objective lens and the eyepiece are graduated lenses (Reticle), on which are graduated lines as thin as hair that cross each other (hence the name "crosshairs"). The lines and scales help the sniper to aim effectively