There are two different processes to "seal" tile. One is to seal the grout joints and other is to seal the tile itself. Which one customer needs, just depends on the type of tile one has...

For basic glazed ceramic tile, it is only required for grout joints to be sealed. This will make them more water and mildew resistant, and help keep dirt out of the joints. Most grout manufacturers suggest to wait a couple of weeks before sealing, so the grout has a chance to thoroughly set.

Apply a silicone or water-based grout sealer to the joints. As long as the tile is glazed it's ok if you get some sealer on the tile surface, but you'll need to wipe it off thoroughly before it dries.

If a porous material like slate, marble or terra cotta is being installed, one should seal the surface of the tiles to prevent them from getting stained with use.

Apply an acrylic top-coat on the tile surface. You'll have to reseal the tile occasionally as it wears off (two or three years after initial sealing). However, we recommend to our customers not to use any cleaners with ammonia on the tile because it will strip the sealer off even faster.