It depends on what kind of glaze was used on the piece, some glazes can be eaten off of while others can not. Lead glazes used to be very popular but recently scientists and doctors have discovered that lead is poisonous to a person's body. A raw glaze, a glaze that hasn't been fired to the right temperature, can be very dangerous to someones health if eaten off of. Any glaze that doesn't reach the correct temperature is known as a raw glaze.
Non-edible glazes are usually only used on decorative pieces such as vases and sculpture pieces, pieces that wont be eaten off of. Potters that make their own glazes have to be especially careful because some chemicals aren't food safe, so they have to make sure the glazes they make can be eaten off of. Most glazes that come mixed together and ready to apply say on the bottle whether or not they can be used with food.