The binder foot is used to apply pre-folded bias binding tape or bias tape you have cut yourself to the edge of fabric in one easy step. It is equipped with a small funnel to fold and guide the binding over the fabric edge before it reaches the needle and can be used with either zigzag or decorative stitches as well as straight stitch, 

Janome

Memory Craft 9900, Memory Craft Horizon 12000, Memory Craft Horizon 14000, Memory Craft Horizon 15000, Memory Craft Horizon 8200, Memory Craft Horizon 8900, Memory Craft Horizon 9400, Skyline 5, Skyline 7, Skyline 9