"In "military-speak," the term "click" (spelled with a "c" instead of a "k") is used when sighting-in a weapon, such as a rifle. On most weapons, one "click" equals one second of arc, or -- in other words, one inch of distance at one hundred yards. So, moving the site adjustments of the rifle "one click" will change the point of impact one inch for a target 1,000 yards away, two inches for a target 2,000 yards away, and so forth. The term comes from the clicking-sound made by the sight adjustment knobs as they are turned."
U.S. Military FAQ -- How Far is a "Klick" in the Military?