The Module Brower is an interface for currently loaded and available modules in GIMP, along with information about the module. The current GIMP uses modules only for color selectors (there is also a gamma display module). In the module browser you can set if these modules should be loaded at start up (this button is non-functional at the moment and all modules will be loaded by default).
You can also unload a module on the fly, e.g. you may not want the watercolor color selector. If you unload a module, the functionality provided by the module will no longer be available in your current GIMP session. To enable the functionality again, you have to mark the module and reload it.