It is the part under the needle plate, where the teeth are used to move it forward. It is the feeder mechanism which is typically used to pull fabric through a sewing mechanism. A set of feed dogs looks like two or three short, think metal bars, crosscut with diagonal furrows, which move back and forth in grooves slightly larger than the bars. The type of motion used serves to pull the fabric through, since the “dogs” are in contact with the material on the forward stroke, and then are pulled back down below the main plate on the backward stroke by the sewing machine’s mechanism. The result is that the fabric is pulled along in slight steps.