A glazed tile has a thin layer of glass fused to its face during firing. The glaze gives the colour, the pattern and the gloss or matt finish, and it seals the surface. Standards mark glazed tiles with GL.
An unglazed tile (UGL) has no such layer: what you see is the fired body. Its colour runs through the tile, so wear does not change its look. Unglazed porcelain is common where traffic is heavy.
The distinction matters for testing. Surface abrasion classes (PEI) apply to glazed tiles only; unglazed tiles are tested for deep abrasion instead.