The spring guide rod is a part that is inserted on one end of the airsoft main spring and is secured on the rear of the airsoft gear box shell. What it does is hold the mainspring in place and prevent it from flexing too much when it is compressed.
They can come in different styles that depend on the model of the gearbox shell that they install into. The two compared below are version 2 and 3 guide rods for version 2 and 3 shells. The version three guide rod has wider ears that correspond to the gearbox shell it goes into.
Many come with thrust bearings to relieve tension from the main spring at it is continuously compressed and decompressed during the compression cycle.