[Rick Parker]

Copied (and edited) from Holley site but says it pretty well, regardless of carb type....Webers or Holley

The main or high-speed air bleeds (Air Correctors in case of Webers) affect the entire range of the main-metering system. The purpose of the main metering system and main air bleeds is to emulsify the fuel before entering the discharge nozzle to be discharged into the air stream in the venturi. The fuel/air mixture
becomes leaner as air bleed (Air Corrector) size is increased. Decreasing the size of the main air bleeds (Air Correctors) will decrease pressure across the main jet, which in turn will pull more fuel through the main system creating a richer fuel/air mixture. The main or high speed air bleeds also act as an anti-siphon or siphon breaker so fuel does not continue to discharge or dribble into the venturi after airflow is reduced or stopped. At high speeds the fuel/air
mixture must be on the rich side to prevent damage to the engine.
The idle system supplies fuel at idle and low speeds. The idle system requires a richer mixture than at cruise speed. Unless the idle mixture is
richer a slow and irregular combustion will occur known as a rough idle. Decreasing the idle air bleed size richens the idle mixture by increasing
the pressure drop in the system. Increasing idle air bleed size leans the idle mixture by reducing the pressure drop across the idle air bleeds.
The same conditions can be created by backing out the idle mixture screws, which will increase the pressure across the idle air bleeds, pushing
more fuel from the idle well creating a richer fuel/air ratio. The Idle mixture screw is the only adjustment recommended for controlling the idle
fuel/air mixture richness or leanness.