A trap for dirt grime soil grease and dirty liquids. accumulates over time and gives the floors a bad appearance most grout lines very porous and can become soiled quickly. Once the grout lines are deep cleaned and restored the will need to be sealed with good quality penetrating sealer. the sealer will protect the grout lines from becoming soiled in the future and much easier to maintain. This is a picture of a typical travertine stone in need of cleaning and restoring.