Are the devices on the front of the machine head that controls the tension on the needle thread. Each needle and thread position has its own Tensioner and all of them should be set as close as possible to get quality stitching. The thread tension assembly normally consists of a post, thread tension knob, tension spring and two tension discs. When the thread tension knob is turned clockwise, it screws down on the post compressing the tension spring on the tension discs. This applies more pressure on the needle thread that is sandwiched between the two tension discs and is against the post. More tension will pull more needle thread from the underside allowing more bobbin thread to be seen.