It is where the carburetor controls the flow of air and fuel into the engine allowing it start up when the engine is cold. It does this automatically instead of having to do it yourself.
I suspect your autochoke is faulty, I has a similar problem a few years ago on a different car, but the symptoms were the same, it's a garage job I'm afraid
It's basically like an autochoke, except along with the pressure regulating the flow, each piston also has a tapered end that the fuel enters and as it rises and falls, which auto-adjusts the amount of fuel.