Grout haze refers to the residue that is left behind on tile after you grout. If you don’t wipe off all grout completely from tile when grouting, grout haze will form and can be difficult to get rid of afterward. It is most visible when the light in the room hits it just right and will appear as a cloudy film on your tile.
To avoid this, work in small sections and be careful to wipe down the tiles really well. Check your tile from different angles because it can be hard to see leftover grout residue in certain light. As soon as the area dries, check it again to see what you missed as it can be harder to see when the tile is wet.
If you are having trouble removing all of the haze, you can purchase a grout haze remover like this one from Rock Doctor to help with the job.