A single-speed bike can have a freewheel mechanism which allows the rider to coast while keeping the pedals stationary. A "fixie" or fixed-gear bike has the gear solidly affixed to the rear wheel so if the wheel turns the gear turns, the chain turns and the pedals turn ---so you can never ;'coast." You always have to be pedaling as fast as the wheel is turning or pretty close to that speed.

This requires some unholy pedaling cadences on downhills.