The yellow light acts as a transition between a green light and a red light. What lies between go and stop? In practice: go really, really fast.

Whereas other traffic lights tell you what to do in clear, unambiguous terms, this one means "proceed with caution." In other words - think about it, keep your eyes open, use your best judgement, etc... Given the view that most traffic engineers have of the general public's driving abilities, this is tantamount to a death wish.