Bridging an amplifier is often most useful when you have a 2 channel amplifier you want to run as a mono channel or a 4-channel amplifier that you want to run as a 2 channel amplifier or if you want to combine the rear channels to a single channel. Bridging is the combination of the left and right channels of an amp, using one terminal from each side, to make 1 (mono) channel with a higher output than either channel running separately. Remember when you bridge an amp the impedance is going to be different so make sure you are using the correct subwoofer or speaker. If the amp can play at 2Ω per channel, when you bridge the channels they will be at 4Ω.
A mono-block amplifier has only 1 channel and cannot be bridged. The 2 outputs of a mono-block amplifier are there to easily connect multiple subwoofers. If you're not sure whether your amp is a mono-block or a multi channel, look at the speaker terminals. Multi channel amps are labeled left and right, and mono-blocks are labeled speaker 1 and speaker 2.
Every amplifier has different requirements for bridging, so consult your manual before making any connections.