This is the traditional pad used on Boehm clarinets. The name is misleading as it doesnt contain fish skin. It is made out of a piece of cardboard with felt on top and is covered by one or two layers of dried and cleaned outer membranes from a calf's intestine also known as goldbeater´s skin. This is the same material used in sausage skin. The pad is soft and usually glued in with shellac or hot glue gun like most other pads. It is cheap and easy to install. Due to its soft surface it will work even when it is not properly seated though spring tension or harder finger pressure will somehow make it seal. You can also find instruments with very strong pad seat impressions. This is usually a sign of bad artisanship. These pads will soon be noisy, since they will touch a larger surface on the tone hole, and dirt between the tone hole and the pad will prevent the pad from closing properly.
As an organic material, the lifetime is very limited (1-2 years). Since fish skin is not airtight, this also reduces the projection of the instrument. The pad will be soft and quiet in the beginning, but after it has been exposed to saliva, the skin will become brittle and vibrate, causing strange production noises. The soft surface also absorbs the sound and is non-projecting which results in a diffused and smaller sound compared with other materials, namely cork.