In the context of the final drive ratio (rear end), it's the number of turns the driveshaft makes per turn of the axle. So, on a 3.92:1 rear gear, the driveshaft will turn 3.92 times for every one revolution of the axle/wheels.
Going to a lower (numerically higher) gear will increase your RPMs at a given speed but improve your off the line performance. A higher (numerically lower) gear will get you off the line slower but keep your RPMs lower at a given speed.
Changing your tires will also affect the "effecitve" gear ratio. A taller tire will have the effect of going to a higher gear - this is why folks will go to 3.92s or even 4.10s when moving up to much larger tires than stock. A lower gear counteracts the taller tires to a degree.
Easy way to remember: High number, high RPMs, high "kick". Low number, low RPMs, low "kick".