This photo shows the underside on an english saddle. These padded channels are called panels and should feel firmly stuffed but feel flex when you press down on it. It should be smooth, not lumpy or overstuffed in some areas and not others. The best saddles are stuffed with pure wool, of varying qualities, but cheap saddles are reportedly stuffed with newspapers, sawdust, or worse. Recently some saddle companies have created saddles with memory foam panels, but the merits of this material for saddles hasn't yet stood the test of time.
Panel restuffing is relatively easy fix for most saddle repair shops and buying a saddle in need of panel restuffing may be a good way to find a bargain. Panel restuffing can be done by any tack shop that does repairs on english saddles and typically costs from $75-$200.