3.5mm audio splitter cable.
It allows you to connect two audio devices, like headphones or speakers, to a single 3.5mm audio jack on a device such as a phone, laptop, or MP3 player.
This specific type of splitter has one male 3.5mm connector that plugs into the source device and two female 3.5mm jacks for connecting two output devices.
Some versions also include a microphone extension, allowing for both audio and mic signals to be split.
These are commonly used for sharing audio with another person or connecting separate headphones and speakers to a single output.