Vitrification occurs during the firing when the clay is physically changed. When the clay is changed it melts the glass formers in clay which allows it to become impervious to water. A clay that is fully vitrified is dense and strong, and bacteria and bacteria cannot grow on it.
When your clay is mature and fully vitrified your clay should not absorb any water.
Porcelain clay bodies will often have no absorption meaning that the clay is fully vitrified when fired to the appropriate temperature/cone. Stoneware clay can’t really be fired to zero porosity due to organic compounds, releasing as gasses later in the firing. Earthenware clays do not contain the necessary amount of glass formers to become fully vitrified. This is the reason it will still be porous even when fired to maturity. Low fire clay can be food safe when a food safe glaze is applied and fired to the correct temperature. Clay and glazes are sufficiently melted together to form a waterproof surface resulting in a glaze coating where food will not penetrate through the surface.