A Monument Sign is a type of sign that is mostly used as large entrance signs or directional signage.  These types of signs would generally be your main road frontage sign.
Samples of monument signs include signage installed on brick or rock structures, custom faricated metal cabinets or larger lighted signs.   Other signs may be used for your main sign, however they may be smaller than what you would typically refer to as a monument sign.  Monument signs are generally permanent structures that cannot be easily moved.