FFP (First focal plane) plane scopes are more specifically designed for tactical shooters and hunters where the distance to target changes and is often unknown. FFP scopes place the reticle in front of the erector. This affects the reticle by changing its size as you move through different magnification ranges.
SFP (Second focal) plane scopes take the reticle and place it behind the erector and right next to the magnification ring on most optics. This affects the reticle by keeping it the exact same size as you move through different magnification ranges. The MIL and MOA scales are only accurate at one specific power and makes it difficult to use Mil or MOA scales at variable magnifications.