There are times where the grout (material used to fill and seal the lines between tiles) gets either dirty, or completely washed out. Usually, this happens when the grout gets worn from age, and gets porous, and when this happens the best thing to do is to re-grout. If you neglect this for too long a time, eventually the water gets behind the tiles, weakening the wall, causing the tiles to break loose from the wall. This process is when you dig out most of the existing grout, and replace it with fresh grout which fills and seals the areas around the tiles. The entire process can be completed in several hours, and makes your tile walls look like new. The entire process can be completed in less than one day, and be used again in 24 hours.