A kenning, in literature, is a word or phrase that is a metaphor for something simpler. Calling a ship a "sea-steed," for example, is a kenning.
You're most likely to hear the term kenning in a literature class, especially if you happen to be studying Old Norse or Old English poetry. It's part of both literary traditions to use figurative language — often in the form of a compound word or a phrase — to represent a simple word. In Old Norse, a typical kenning is "sun of the houses" for "fire." The root is the Old Norse kenna, "know, recognize, or perceive."