Water that has entered a porous tile expands when it freezes and can crack the body or lift the glaze. ISO 10545-12 tests this by soaking tiles and cycling them between +5 °C and −5 °C a hundred times, then checking for damage.
Tiles with very low water absorption, such as porcelain, pass the test as a rule. Outdoors, the whole build-up matters: slope, drainage, adhesive and joints all have to keep water from standing under the tiles.