The 3.5mm adapter allows the user to plug in an external 3.5mm microphone into Osmo Pocket for higher-quality sound recording.It's recommended to use audio-technica atr-6550, Rode VideoMic Pro, Rode VideoMicro, Azden sgm-990, Sennheiser MKE400, Sony ecm-ds70p, DJI fm-15 FlexiMic and other mainstream brands with 3.5mm microphones.