sealant for grout
You follow the instructions on the sealer; which will basically tell you to brush a thin even coat of sealer on the the grout with the applicator, let it set for approx. 5 minutes and wipe off excess with a clean towel. You can tell the sealer is ready to be wiped off when the sealer looks like a matte finish as opposed to a shiny, wet finish. Your best bet is to get as little as possible on the tile. It wipes off pretty easy if you don't leave it on too long. Do a section at a time and you will have no problem.
If you have tile in your shower, you will need to seal the grout there as well.