Grout is a thick substance or paste that is formed with the combination of sand, water and cement. This paste is spread over and between the tiles, once the tiles are put in place on the floor or the walls. Although the tiles stick to the floor or wall with the help of cement, the white (sometimes colored) grout is used from the top to hold the tiles together with each other and to fill up the spaces between the tiles with a neutral colored paste. It gives a clean finish to the whole surface and it also prevents water or any other substance from going underneath the tiles.
Grout is porous and allows moisture and dirt to penetrate it. The build up of the dirt and moisture in the porous grout means in the worst circumstances it can turn black as well as harbour odours related to these areas.