The final step in tiling your outdoor patio is to seal the tile grout. There are many sealers (or "sealants") on the market used to seal tile grout, protecting it from stains and prohibiting moisture absorption, which can be a problem in freezing conditions.
Most home improvement stores carry a variety of tile grout sealers. The most common are penetrating sealers and topical sealers. Topical sealers only seal the top layer of the tile grout. They're very effective at keeping tile grout clean, but only until foot traffic works the sealer off the surface. Penetrating sealer seals all of the tile grout. While a bit more expensive, it will keep your tile grout looking good longer.
Each sealer is applied differently. Follow the manufacturer's instructions. Pay particular attention to the amount of time the tile grout must be allowed to cure before the sealer can be applied.