Some understanding of bullet trajectory and the physical factors affecting bullet flight is needed as background before discussing optics.
In the simplest case, take an accurate rifle with sights zeroed at 100 yards shooting one type of ammunition. In the absence of wind or shooter error, the bullet will impact the point of aim (POA) when the target distance is 100 yards-- hence its "zero" is at 100 yards.
The "line of aim" is a line straight from the shooter's eye, through the sighting device, to the target. The bullet starts off below the LOA by the distance between the center of the sighting device and the center of the bore. This is called the "sight over bore" distance. The axis of the bore is not parallel to the LOA-- the bore is angled slightly upwards. This causes the bullet to start off with some "upward" velocity. As it flies down-range, it rises to meet the point of aim (POA) which is where the LOA intersects with the target.
Depending on the bullet's velocity, the bullet might keep rising above the LOA and again intersect with it a second time as it falls. Alternatively, it may rise just enough to meet the LOA and then start to fall again.
In this graph, two loads are displayed. The green trajectory is a 308 load zeroed at 100 yards. It starts 2" low, rises to the LOA at 100 yards, and then drops off the graph 8" low at 267 yards.
The red trajectory is the same load zeroed at 200 yards. It starts 2" low, intersects with the LOA the first time at about 40 yards. At 120 yards, it's about 1.6" above the LOA, then drops, intersecting the LOA again at 200 yards. This is the second, or primary, zero. At 300 yards, it's about 7" low.
Looking at the graph with the 200 yard zero, the point of impact (POI) at 100 yards would be about 1.6" above the point of aim (POA). At 240, the POI will be 2" below the POA. At 300, the POI will be 7.5" below the POA. Thus, to hit a small target at 300 yards, the shooter would have to hold 7" above the target. The bullet continues to fall relative to the line of aim as target range is increased.
A table can be constructed which relates the drop distance for every range out to the maximum engagement range. An abbreviated table might look like this, for a rifle with a 100 yard zero. (An actual table would have intermediate distances like 120, 140, etc.)
This is helpful, but the shooter is left with the problem of how to aim 47" higher than the target when the distance is 500 yards. There won't be a 47" yardstick sticking out above the target. Aiming the cross-hairs at a point imagined to be 47" above the target is difficult and very error prone.