If your overflow hose is routed to that general area and your coolant reservoir was topped up lately, it may be nothing. If not, it may be that your water pump is shot. Less likely, but still possible, is a bad water pump gasket.
It's a huge pain to look for, but there should be a weep hole on the bottom of the water pump that leaks when the seals are done. If coolant is coming from the weep hole, you need to replace the pump. If coolant is leaking from the interface between the pump cover and the pump itself, you probably just need the gasket.