Grout is a colored cement mixture used to fill in and connect the voids between tiles. There are 3 main types of grout; sanded, un-sanded, and epoxy grout. All but, epoxy grout can benefit from sealing. The most common type is sanded grout that is used for grout joints 1/8 inch or bigger. Sanded grout consists of Portland cement and sand. Sand is used because it helps prevent the grout from shrinking and cracking after drying.
The mixture of sand and cement makes the grout very porous, ready to absorb any liquids that get on it. This becomes a problem when the liquid is an accidentally spilled coffee, wine, or juice that gets inside the porous grout. After your spilled coffee or juice penetrates the grout the water will evaporate leaving the color content of the coffee or juice behind. This leaves a permanent discoloration called a stain that will make your floors look old and worn. The only way to remove this stain is by removing the old grout or having to color stain all your grout into a different color. This process is tedious and expensive.
To avoid having discolored grout and protecting and preserve your perfectly even colored grout color from any stains or discoloration a sealer is applied. The most common type of grout sealer is a penetrating sealer. After application, the sealer penetrates the grout, and later the liquid content evaporates leaving behind a protective coating inside, that will help prevent any stains.
Depending of what you need there are several types of grout sealers out there; some penetrate the grout others stay on top of grout, some darken or enrich your grout color while others do not change the appearance of grout at all.
Sealers do wear off over time and the grout should be resealed when the manufacturer recommends.
So the next time you spill a cup of coffee, juice, or any colored drink, the sealer will give you enough time to wipe the spill before it penetrates the grout. If the spill is left on sealed grout, overtime the liquid will penetrate the grout through the sealer and stain the grout.
When you do wipe the spilled drink in a timely manner, the sealer will do its job and prevent any discoloration or stains from forming.