What is a 3.5mm stereo jack socket?

It's the smaller of two standard connectors used for headphones and other devices. Until recently, its size was described in inches -- 1/8", with the larger size being 1/4" (6.3 mm). The measurement refers to the diameter of the male plug. The smaller jack is typically used in phones and other small portable devices. The larger is typically used in large over-the-ears earphones, and, in mono form, to connect electric Guitars to amplifiers, and the amplifiers to speaker boxes.