The main function of a carburetor is to mix air and gasoline in the correct ratio to ensure proper engine function. If the carburetor isn't tuned properly, the engine receives the wrong fuel-to-air ratio, which can lead to poor performance and possible engine damage.
A properly tuned carburetor is one in which the fuel-to-air mixture is maintained at the proper ratio regardless of the position of the throttle. When not enough fuel is allowed through the carburetor, the engine "runs lean," which can cause serious engine damage. When too much fuel is allowed through the carburetor, the engine "runs rich," which can cause a significant loss in engine performance. The carburetor, while still in use in many household objects with internal combustion engines, has been largely supplanted in the automotive world by fuel injection.