A kenning is a poetic device used in Old Norse and Old English poetry where something is described indirectly using descriptive phrases rather than directly stating what it is. The summary provides examples of kennings such as "heat giver" for fire and "light maker" for the sun. The document teaches how to write kennings by thinking of a subject and describing what it does using two-word phrases. It then provides examples of kennings for various subjects like a dog, ball, parent, and teacher.