Meaning

• Any of various substances designed to extend any of several properties of a material.

• Any of various components designed to extend the length of a device.

• Any substance added to food to bulk it out, with a higher protein content than a filler.

  "Soy protein is used as a meat extender."

• A runner, or quick-draw. (from sense #2)

• A phrase that extends an expression to include further members of a set, e.g. "and stuff", "or something".