the numbers correspond to the revolutions of the drive shaft compared to the tire. If you have a 3.73:1 rear end ratio, that means that the driveshaft/pinion gear will spin 3.73 revolutions to the 1 revolution of the tires/ring gear.
when talking about gear sets they are backwards. the higher the number the lower the gear and vice versa.
3.42 is a "high" gear whereas 4.10 is a "low" gear
Higher gears are good for highway use and all out speed. but are terrible for pulling and take off.
lower gears are good for pulling (it acts as a torque multiplier) and getting out of the whole but are terrible for top end.