A kenning is a metaphorical phrase used to replace a noun, often using two words to describe the noun indirectly. In this case, "fiery-footed" is a kenning for "god", particularly referring to Thor, the Norse god of thunder, who is often depicted as travelling in a chariot drawn by goats with fiery hooves.
Here are a few other examples of kennings:
* Whale-road for "sea"
* Battle-sweat for "blood"
* Sky-candle for "sun"
Kennings were frequently used in Old Norse and Old English poetry.