Grout needs to be sealed immediately after it dries and cures. Sealant protects the surface from moisture and dirt. You should reseal grout lines every year, since the seal wears away over time. To test to see if you need to reseal yet, sprinkle water over the grout lines. If it beads up, the seal is still working. However, if the water soaks in, the porous surface is exposed, and you need to reseal it.