What does fire mean in the context of “divided tongues, as it were of fire” (Acts 2:3)?

Acts 2:3 (YLT):

3 and there appeared to them divided tongues, as it were of fire; it sat also upon each one of them,

What does fire mean in this context? And how is the fire related to the divided tongues? How should we make sense of the individual words and the full expression as a whole?