Grout is a thick substance or paste formed with the combination of sand, water and cement and used as a construction material to seal the spaces between tiles or concrete. 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 coloured) grout is used to hold the tiles together and to fill up the spaces (seal) between the tiles with a neutral coloured paste. It gives a clean finish to the entire surface and also prevents water or any other substance from seeping beneath the tiles.
Grout is porous meaning that it can easily be penetrated by moisture and dirt. The build up of dirt and moisture in the porous grout means in the worst circumstances it can turn black as well as harbour odours and bacteria.