A kenning is a type of compound metaphor which is very commonly found in Anglo-Saxon and Old Norse poetry and prose. The etymology of the word itself gives us some indication of how these conventional phrases were used: "kenning" comes from the Old Norse "kenna," which meant "to know." This word survives in the modern English phrase "beyond our ken," meaning beyond our understanding, and is cognate with words like "cunning" and "canny." The point is, a kenning was something the reader was expected to know and recognize, even though it doesn't describe something directly. Most kennings would have been used so often that readers would associate them immediately with what they represented.
Examples in Beowulf include the very famous "whale-road" in line ten, "hronráde" in Anglo-Saxon. This means the sea, as in a road traversed by whales.
Another great example, used several times, is "béaga bryttan" or "giver of rings." This means a lord and helps us to understand the ceremonial relationship between lord and vassal in Anglo-Saxon culture, which was solemnized by the giving of a ring. This was supposed to represent the lord's acceptance of his duty to provide fiscally for the vassal.
Later, we see an interesting kenning, "beorht béacen godes," used to describe the sun. This literally means "bright beacon of God" and is interesting because it contributes to the debate around the Christianity of Beowulf and whether the Christian elements were added later. This kenning indicates that the Anglo-Saxon language, at least, continued to accumulate new kennings long after Christianization.