In simplest terms, "bridging" an amp means that you're taking two
output channels from the amp and combining them into one output
channel.  The one resulting channel has the potential to provide four
times  as much power as one channel did before your bridged the amp.

Here's an example.

You have an amplifier that can provide 25 watts per channel into two
channels, with each channel driving a 4-ohm load.  The output voltage
for each channel will have a 10-volt range.  Since power is voltage
squared divided by load impedance, each channel is (10 X 10)/4, or 25
Watts.  Since you have two channels, the amp can provide 50 Watts
total.

Now let's bridge the amp.  When the two channels are combined, the
total output voltage goes from 10 volts to 20 volts.  Power is still
voltage squared divided by load impedance, so for a single 4-ohm load,
the output power becomes (20 X 20)/4, or 100 Watts.  The amp has gone
from providing a total of 50 Watts (25 times 2 channels) to 100 Watts
(100 times 1 channel).

Not all amplifiers can fully double their output power when bridged -
it depends on if it can provide the extra current required.  I have a
Precision Power A1200 that will do 300 X 2 or 1200 X 1, so it can
fully double its power, but my old Linear Power 2202IQ would do 110 X
2, but only 375 X 1, instead of the 440 X 1 it would do if it could
truly double its power into a bridged load.

Scott Gardner