A quick search on Google Groups told me what they are:

It’s basically a weight about the size and shape of a hockey puck of just a few grams suspended inside a cylinder full of fluid, such as hydraulic fluid. When the nose bounces up and down, the weight slides up and down opposite of the nose in the cylinder creating a harmonic counter frequency to the nose bounce.

Apparently they’re similar to the tuned mass dampers that you have in skyscrapers that when hit by strong winds or other forces (like earthquakes) move in the opposite direction to help counter balance the affect. Clever stuff.