Tile glaze is the shiny topcoat of clear gloss that gives it its shine. Clay-based tiles like ceramic and porcelain are often manufactured with a glazed top (it's baked on), but it can wear down or get scratched over time. One option for restoring that glaze is to use floor-grade polyurethane gloss, the same material you would use to finish a hardwood floor. It's important to lightly sand the gloss between each layer so it adheres properly.
Remove the old glaze, using a vibrating sander and medium-grit sandpaper. Wipe up the dust with a clean, soft cloth.
Brush polyurethane gloss on the tiles with a paintbrush, starting at one corner and working your way out. Keep the brush strokes all going in one direction, and do not let drips or bubbles form. Allow the polyurethane to dry completely (four to six hours).
Buff off the shine with fine sandpaper, sanding it by hand in light, fast strokes. Wipe up the dust.
Spread a second layer of polyurethane in the same manner as the first. Let it dry.
Sand the second layer of polyurethane in the same manner as before. Clean up the dust with a soft cloth. Apply a third and final layer, getting it completely smooth and shiny and letting it set for at least two days before resuming use of the area.
Wear a dust mask when sanding the tile.