instead of using both channels on a 2 channel amp, you will combine them by only using a positive output from one channel and a negitive output from the other channel. depends on the amp though, it'll tell you which ouputs to use to bridge it.
you'll cut down the number of outputs from a multi channel amp and gain a little output.
but as TY said, most bridged amps are not stable below 4 ohms, so it'll depend on your subs as to wheather you can use a bridged setup or not.