The spring guide is a pretty self explanatory part - it holds the spring in place and ensures that it compresses in the right way i.e. doesn't bend into the gears etc. This particular one is for V2 gearboxes, and is manufactured by SHS. Constructed out of stainless steel, and complete with bearings, this spring guide is superb.
Due to the bearings this part can also bump up the power in your gun a little, as it increases the spring's compression, so bear that in mind. As always fitting upgrade parts may require some modifications as there are small differences and deviations in size across brands & models.