Bridge mode is commonly used to get more output power from amplifiers by combining two single channels output into one output. This can only be done safely if you know the minimum load (Ohms) permissible when bridged. This is more common when driving Subwoofers that have Dual voice coils or when you have multiple Subwoofers. First check the installation manual or look at the speaker terminal block on the amplifier as they normally have a diagram showing the bridged connections.
This is an example of a 4 channel amp speaker connection terminal block. As shown bridging is achieved by connecting the positive terminal from the left channel and the negative terminal of the right channel to the corresponding terminals on the subwoofer coil or coils depending on configuration. There are a number of different configurations depending on the load applied to the amplifier. See Subwoofer coil configuration. It all depends on whether you have a 4 Ohm, 2 Ohm or 1 Ohm stable amplifier (mono). An example of the different power ratings for a four channel amplifier are as follows.
100 Watts RMS x 4 @ 4 Ohms
200 Watts RMS x 2 Bridged @ 2 Ohms
These are examples only to give you an idea how this works. This also applies for 2 channel amplifiers and depends on whether they class AB or D class amps as the 1 Ohm rating applies to D class amps in general.