In layman's terms, that means that, when it's on the index setting (assuming you have everything adjusted right), there are a series of "clicks" in the lever (you can both hear and feel them), and it will shift one gear over every time that you push the lever to the next click. The friction setting lacks those "clicks", so you have to push the lever until you hear the gear change and then you stop pushing. With time and practice, you get a feel for how far to push without listening or pushing too far and the friction setting lets you shift even when your adjustments aren't perfectly spot on. Some people prefer one, others prefer the other.