Lower numbers mean a higher effective final drive. So for a given transmission gear at a given RPM, a 3.08 will be going faster than a 3.42.
You'll read about "taller" gears: taller = lower numbers. a 3.9 is "taller" than a 4.22.
For instance:
at 3500 RPM in 4th gear with a 4.22 rear end I may be going 55 mph
at 3500 RPM in 4th gear with a 3.90 rear end I may be going 60 mph
That's what I know. I'm sure smarter folks will add more...