Feed dogs

Feed dogs are the critical component of a "drop feed" sewing machine. A set of feed dogs typically resembles two or three short, thin metal bars, crosscut with diagonal teeth, which move back and forth in slots in a sewing machine's needle plate. Their purpose is to pull ("feed") the fabric through the machine, in discrete steps, in-between stitches.
This arrangement is called "drop feed" in reference to the way the dogs drop below the needle plate when returning for the next stroke. Allen B. Wilson invented it during the time period 1850 to 1854, while also developing the rotary hook. Wilson called it a "four-motion feed", in reference to the four movements the dogs perform during one full stitch: up into the fabric, back to pull the fabric along to the next stitch, down out of the fabric and below the needle plate, and then forward to return to the starting position.
Virtually all drop-feed sewing machines can vary their stitch length; this is typically controlled by a lever or dial on the front of the machine. They are usually also capable of pulling the fabric "backwards", to form a backstitch.