A carburettor is a device which prepares the air/fuel mixture needed for combustion in an engine. Carburettors are now almost obsolete, and have been replaced by fuel injection.
A restriction or venturi is core to the operation of a carburettor. A fuel supply pipe emerges into the venturi, which carries intake air to the cylinders. As the air passes through the venturi its pressure drops, and this low pressure sucks fuel from the supply pipe. The fuel and air mix thoroughly in the intake system before they reach the combustion chamber.
The drawback is that it is very difficult to design a 'perfect' carburettor which supplies the ideal air/fuel mix for all engine operating conditions (in fact the carburettor was once defined as an instrument which provides the wrong mixture for all conditions). Fuel injection, which allows more precise control over fuelling, has largely taken over from the carburettor.
