He means the part you see if you take the abs sensor out, there is a toothed ring, called the stator, which is what the sensor is actually sensing, it makes the sensor produce a sine wave that the abs computer processes into the wheel speed. If the stator gets too crudded up it can make a "ratty" signal for the sensor.
edit, probably not the best example, but this shows what the abs sensor is pointing towards, I think it is just a machined part of your hub, not a seperate part:
