TRRS connectors are the audio-style connectors that you see on some phones, MP3 players and development boards. TRRS stands for "Tip, Ring, Ring, Sleeve," which reflects the fact that, unlike a standard stereo connector, this actually has three conductors and a ground. Some devices use the extra conductor for a microphone (like hands-free headsets) or to carry a video signal (like in some MP3/MP4 players). If you're hacking on something that has a 4-conductor audio jack, being able to plug straight in will keep your build clean and simple.
Note: This plug allows you to add a custom length of wire to your connector or repair a broken shroud on an existing cable you just cannot live without. Unfortunately, the supplier has not provided a datasheet or dimensional drawing please see the similar documents provided by other SparkFun products on the "documents" tab.