Bridging is when the saddle touches at the withers and under the back of the cantle but there is open air under the center of the saddle. When the rider sits in the saddle, it jams the front and back of the saddle tree into the horses back causing significant pain. Not much can be done to help this issue short of a new saddle. Sorry. Much easier to fit a rocking saddle temporarily.
The description above is accurate. However, depending on how severely it is bridging (could be anywhere from the above description of "air" under the saddle or just less pressure in the center than the back and front of the panels), a saddle fitter can work on it if it is wool flocked and should be able to fill in the panels to even out the contact.