It is typically used to connect headsets with separate headphone and microphone connections to new laptops or ultra books equipped with a single.

3.5mm 4-position audio port.
3.5mm (3 Position) Female - Green - Headphones
3.5mm (3 Position) Female - Red - Microphone

Features:

• 3.5mm TRRS Stereo Male to Dual 3.5mm TRS Stereo Female Y-Cable
  
• Use to connect computers with a single 3.5mm 4-position audio port to a headset with separate  headphone and microphone connections
  
• Fully molded connectors provide strain relief
  
• Foil shielded to prevent external EMI/RFI interference
  
• Stranded conductors and drain wire are tinned to resist oxidation