Center Plane of a Feature is the center plane of an unrelated actual mating envelope or actual minimum material envelope of a feature.

Derived Median Line is an imperfect line that passes through the center points of all cross sections of the feature. These cross sections are normal to the center plane of the unrelated actual mating envelope.
Derived Median Plane is an imperfect plane formed by the center points of all line segments bounded by the feature. These line segments are normal to the center plane of the unrelated actual mating envelope.
Modifiers communicate the additional information about the drawing or tolerancing of a part.
Maximum Material Condition (MMC): Maximum material condition is the condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size- for example, the largest shaft diameter or smallest hole diameter.
In simple Words, maximum material condition adds material to the part, increases the weight of the part.      
   

Maximum Material Boundary (MMB): Maximum material boundary is the limit defined by a tolerance or combination of tolerances that exists on or outside the material of a feature(s)
Least Material Condition (LMC): Least Material Condition is the condition in which a feature of size contains the least amount of material everywhere within the stated limits of size for example, the smallest shaft diameter or the largest hole diameter.
In simple Words, least material condition removes material from the part, decreases the weight of the part.