Easiest was to explain it is that an M6 with a 3.42 factory gear reaches 30 mph at 5000 rpm in first gear (estimate). Just changing to a 4.10 will also get to 30mph but at say 5300 rpm (just estimates). Basically you run our of rpm's faster with the higher ratio (4.10) but you accelerate quicker.

The rear gear number is a ratio between the number of teeth on the ring as compared to those on the pinion.

Does that help? It was my attempt.