The most frequently misunderstood is the difference between first and second focal plan. The difference is where the reticule is positioned in the scope this affects the image you see at differing magnification.

focal-plane-vs-second-focal-plane-rifle-scope-reticle

First focal plane scopes are useful when shooting at varying ranges as the stadia remain in the same place on the target when you change magnification . As with the second focal plane only the cross-hairs remain in the same position.