What is a sewing machine feed dog?

Feed dogs are the 2 little "toothed" bars that are under the presser foot. When you operate your machine, they rise up a little, the teeth grab the fabric, and then they move away from you, taking the fabric with them.At the end of each stitch, they drop down below the bed of the machine, move towards you again, come up and grab and pull the fabric along, over and over. This is what causes the fabric to feed evenly through the machine.

The feed dog or dogs in a sewing machine are the little teeth-like things under the pressure foot (the thing you lower before starting to sew) that help to pull the fabric through.

The function of the feed dog device on the sewing machine, is to move the motion of the fabric.
Feet dogs are actually "feed" dogs, these are the teeth that come up through the needle plate to move the material under the presser foot to make each stitch. The feed dogs can be lowered on some machines for free hand embroidery or darning.