When an engine operates at full throttle, that's it's top end.
When it's idling, it's operating at the low end. It's idle performance is it's low end performance.

There are two carburetor designs we see most in our glowfuel model engines. These two designs are basically different ways for the carbutetors to deal with the engine's demands for wide open performance and the different demands for idle or low end performance. The amount of fuel to air mixture if the problem. The engine needs one ratio of fuel/air for top end and a different ratio for low end. The most popular design right now is a carb that controls the intake mixture by using an additional needle valve to change the ratio when the carb is partially closed for the low end performance. The other method deals with the ratio by regulating air into the carb through a hole for the low end performance. Those are called air-bleed carbs. They have an airbleed hole in the front of the carb.

The popular carbs that aren't airbleed.

The ratio of fuel/air can have more fuel to air or less fuel to air. When there is more fuel to air, the mixture is said to be rich. When there is less fuel to air, it's said to be lean.
With an airbleed carb, increasing the size of the airbleed hole lets increased amounts of air into the area where the fuel and air mix, and the mix has more air than it would otherwise, so the mixture is leaned.