A kenning is basically a two-word description in place of a more common one-word noun or name. Kennings are often seen in Anglo Saxon poetry, which was originally transmitted orally. In Beowulf, as in other poems of the time, the poet uses kennings to add variety to the wording and to more vividly paint a picture of the action for the reader.
In the section of the poem where Beowulf fights Grendel, we see a couple of kennings to describe Grendel. The poet calls Grendel a "terror-monger," which is a kenning that conveys the fear evoked by Grendel. Grendel is also referred to as a "hell-serf," which shows that he is a servant and devotee of Satan. Both of these kennings enhance the description of Grendel as evil.
A couple of other kennings are used throughout the battle to describe places or things. Grendel's presence in the mead hall is referred to as "a hall-session," which, unlike the kennings applied to Grendel, seems to de-intensify the event. Grendel's sinews, muscles, or tendons are referred to twice as "bone-lappings," a vivid description of Grendel's physique that adds to the gore and violence of the battle scene.
Beowulf is described as "the earl-troop's leader" in a kenning as well. This kenning indicates his position and contrasts with the more loaded descriptions of Grendel discussed above.
These kennings give the poem more stylistic variety than if the poet had used Grendel's name or simple nouns. The poetic descriptions also enhance the imagery and mood of the scene.