What do I need to know about this rubber stopper? I recently bought a used Diamondback Sorrento.  One day I rode it over a sizable bump and heard a popping noise.  Upon inspecting my bike I noticed the following:



As far as I can tell, I lost a rubber stopper that protects the shocks from dust/water/etc.  What exactly is it called?  How important is it that I replace it?  Do I need to worry about protecting the inside of my shocks until I do?
On your particular shock, it is as you guessed just a cover to keep out dirt. Replacement parts for these shocks are generally not available. I would suggest a visit to a local hardware store and find a generic rubber /plastic plug that will fit in the hole.