from https://www.americord.com/blog/what-is-the-nema-5-15-connector-how-does-it-work/
also see https://en.wikipedia.org/wiki/NEMA_connector with much more info

NEMA = National Electrical Manufacturers Association

Types of NEMA connectors

• Locking. Have curved blades, turn plug after inserting to lock it
• Grounded or ungrounded

NEMA 5-15, NEMA 6-20

• 5 is 125volts, 6 is 250 volts, both grounded. 1 means 125volts, ungrounded
• 15 or 20 is amperage rating, 15 amps or 20 amps
• 5-15P is a plug, 5-15R is a receptacle
• L means locking