Fixed gear bicycles are just like a normal bicycle except, they lack the “free wheel.” The “free-wheel” is the mechanism that allows a bicycle’s pedals to stop rotating allowing the rider to coast. A fixed gear does not have this mechanism, which means the rider cannot coast. This allows for the bicycle to be stopped by resisting the pedal’s forward movement.
It can take some time to get used to riding a fixed gear bike once a rider makes the change. The ability to coast is a luxury that most riders take for granted. The best comparison for those who drive would be that an automatic car is sort of like a free wheel bike while a standard or manual transmission is more like a fixed gear. Each just requires a bit more of the operator’s attention.