Read up on this page;
http://science.howstuffworks.com/gear-ratio.htm
I know it's "geeky" but this site will pretty much cover the basics of how parts in cars work. This explains how gear ratio's work.
When talking about gear ratio's on cars, it refers to the transmission or the differential.
HICAS lock bar is to simply remove the HICAS system on Nissan cars equiped with it. In simple terms, the HICAS is a computer/hydrolic system which makes corrections to the cars rear wheel above 80km/h in a given corner. It has nothing to do with the diff.
For the average driver it works a miracle. But for someone who's looking for complete control of the car, it's not pleasent. The computer is correcting the driver, while the driver is correcting the computer so it's a never ending battle.