When we talk about rich and lean, we're discussing the amount of fuel in the fuel/air mixture that gets sucked into the engine via the carburetor. The needle valve is there to control the flow of the fuel. Open the needle valve (by turning it counter-clockwise) and you'll put more fuel into the mixture--this richens the mixture. The opposite of that is leaning the mixture, which is done by turning the needle valve clockwise, so that there is less fuel involved.

The high-end needle is the big one. Most all of our carbs have high-end needles. As the name suggests, it controls the mixture at the high-end of the throttle settings. About the top 1/3. The rest is controlled by the low-end needle valve, or air-bleed valve on engines that don't have a low-end needle valve.

When you adjust your high-end needle, do it with the throttle all the way open. Start with the needle setting that your engine instructions specify. This is usually a rich setting. You make small adjustments to the needle, one or two clicks at a time. By click, I mean the clicking feeling that the ratchet on the needle provides.

Being too lean is very bad for the engine. Our glow engines get their lubricant with their fuel, so when you don't have much fuel you naturally don't have much lubricant. Being too rich isn't good either, as the engine may not reach it's designed operating temperatures.

The best way to get a feel for what all this really means is to apply it hands-on. Have your instructor go over the terms and then show you how to properly adjust an engine. Get some tips from other sources, too, and find out what works best in your own experience. A properly tuned engine is usually very reliable, and that makes the whole hobby less frustrating and more fun.