It's about how many clicks of elevation you need to "come up" when changing distances. Usually on a target range. For example, with an M1 rifle using M2 ammo, you need to increase your sight's elevation or "come up" 2 clicks from whatever your 100 yard mark is(that varies slightly from rifle to rifle) when moving out to 200.
It's the same idea with a hunting rifle's scope. If you need to raise the elevation by, say an inch at 100, you turn the elevation screw the number of clicks required to do so. That varies by the scope too. IE. a 1/4 minute scope needs to "come up" 4 clicks to raise the elevation one inch at 100 yards. However, most scopes on hunting rifles are sighted in high to eliminate hold over.