A flow regulator is like the petcock valve (or faucet) on your garden hose. It controls the flow of liqiud/air through a device. Turn it a little bit and only small amount comes through but it will continue to flow through until the pressure equalizes on both sides of the valve. That's why you get a blast of water from a garden nozzle when you first squeeze the handel then it dwindles down. The same will happen to your airbrush.
A pressure regulator, well, regulates the pressure that the air/liquid is released at. This is what you want on an airbrush.