Bridging is a term that often people have heard but don’t actually know what it means. If you aren't sure if your saddle is bridging then place it on the horse in the correct position (see the video section on my website for more info on this if unsure) and just lift up the flaps gently and see if you can see daylight underneath.
Sometimes if you look from the pommel through to the panels you can also see it, but depends on the horses’ shape. This means the panels are not contacting the back the whole way through. This also means there will be more pressure front and back.
Often you will find saddle fitters don’t mind a little bit of bridging as they think that when the horse lifts up if will even the pressure out. Even if the horse lifts its back up when ridden there will still be more pressure front and back. Likely you have too narrow a gullet or/and the wrong shaped saddle for your horse.
In the first photo below that shows an example of bridging. In the second photo that show what a talc test will show if saddle is bridging.