NEMA is short for National Electrical Manufacturers Association and this is the certifying body that stands behind electrical connectors used in North America.
The “5” in the NEMA 5-15 designation refers to the configuration of the three blades on the male connector and the corresponding female slots on the other cable end or on the wall outlet.
NEMA 5 male connectors have two parallel flat blades and a third “U” shaped blade that provides grounding for the device plugged in to the cable.
The “15” in the NEMA 5-15 designation refers to 15 amps, the amount of current that the connector is designed to carry without fear of overheading.
You can read more about the NEMA 5-15 connector designation by clicking here.