These are cables used to run an onboard preamp into standard sized 6.35mm (1/4 inch) output cables. They have a 3.5mm male jack (straight or right angle options) with 55cm of cable running to a 6.35mm female jack socket.
3.5mm is the standard size for onboard acoustic guitar preamps. These cables allow you to run the output of your onboard preamp directly to the output of your guitar.