Where as a common stereo amplifier (2 channel amp) each speaker + – is connected to the corresponding channel 1 (L) and channel 2 (R) terminals, creating stereo sound.
In Parallel Mono, the same signal is played through both channels.
In Bridged Mono a speaker is typically connected to the + terminal of channel 1 (L) and channel 2 (R) of the Amp, with one channel acting as + and the other channel as a -. This harnesses the power of both amplifier channels theoretically doubling the output (although its often not quite as much)
To acheive Parallel or Bridge mono your Amp must be capable of switching to each mode, which is often through input connections or a switch on the rear.
Bi Amp, uses 2 or more Amplifiers to drive different sets/sections of speakers on a system, which gives minimal distortion, more dynamic range and a clearer enhanched sound quality.
Bi Amping decreases the chance of blowing tweeter, cross over frequencies are at line level and again more enhanced sound.