It is a high-quality 3.5mm TRRS to TRS adaptor, which enables you to connect your smartLav mics to 3.5mm TRS devices (cameras and audio recorders). For simplicity, gold-plated contacts are colour-coded, with the grey one indicating the TRRS input.
If you record audio on the road and need more convenience when connecting your mics to your computer, Rode's SC3 will definitely come in handy.