it is a ratio of turns in to turns out.
Or for so many turns of the driveshaft you get so many turns of the wheels. For example a differential of 3.92:1 turns the rear wheels once for each 3.92 times the driveshaft turns. Now imagine a 3.69:1 diferential the ratio works the same way, but the difference in the real world is can be best illustrated by this:

At engine 2000rpm with the manual transmission in 4th turns the driveshaft 2000 times in one minute. (4th is a 1:1 in mine and I think yours) the wheels then with a 3.91 will turn 511.5 times. With the 3.69 differential however the wheels will turn 542 times. With an estimated circumference of 6.25 feet for a 14" wheel, the 3.69 will travel 190 feet 7.5 inches further the same car with the 3.92:1 differential. Alternatively, the engine turns less turns to go the same distance with the 3.69 or at a fixed speed will have a lower engine speed than the 3.92. Keep in mind the lower the rear ends ratio the more oomph your engine will need to produce to make the car move. So off the line the 3.92 will be quicker because there is more torque at the rear wheels.

I hope all this got through the keys alright, I have a cold thats turned my brain into a throbbing mush...