To begin, a kenning is an Anglo-Saxon metaphor. The original language of Beowulf did not have the flexibility that English does today, so descriptions were made using a kenning.
One definition of a kenning is:
In Old English poetry, an elaborate phrase that describes, persons, things, or events in a metaphorical and indirect way.
The kenning often uses figurative language and can sometimes (not always) be made up of two hyphenated words as way of a description, or a short phrase. The sea was called a "whale's home" in The Seafarer. There are several richly descriptive phrases used to describe Grendel.
In the translation by poet Burton Raffel, note how the monster is described as he crashes into the mead hall, kills one of Beowulf's men, and is then surprised—and stopped—by the amazing strength of our Geatish hero:
Then he stepped to another
Still body, clutched at Beowulf with his claws,
Grasped at a strong-hearted wakeful sleeper
—And was instantly seized himself, claws
Bent back as Beowulf leaned up on one arm.
That shepherd of evil, guardian of crime,
Knew at once that nowhere on earth
Had he met a man whose hands were harder...
Grendel is also called "the infamous killer," "the Almighty's [God's] enemy," "hell's captive," and "sin-stained demon."
Beowulf is described as "That mighty protector of men." The words he speaks to Hrothgar at their meeting are called "bright-tongued boasts."
Beowulf describes Hrothgar in one instance as "shelterer of warriors."
The language seems not limited as one might think of this ancient language, but poetic and grand, lending itself to the author's tone of bravery and martial excellence.
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.