The shutter module will permit or deny passage of items or fluids in either of the directions of the pipe or side of the machine it is on. This is useful for gregtech fluid pipes especially to prevent 'sloshing' of fluids back through long pipe runs. It can also be useful to allow fluids into machines that otherwise auto-output fluids into pipes (i.e. water or lava into the side of a GT single block boiler, while preventing the machine from trying to output steam into the pipe)
If a Machine Controller is placed on the same block as the shutter module, the shutter can also be opened or closed based on a redstone signal to the machine controller.
For most uses Shutter Modules are obsolete now, as it is possible to shift rightclick pipes to prevent input from the selected side. This can be used to prevent sloshing without the need for covers.