When you install tiles in a shower, you must put grout between the tiles to prevent dirt from penetrating to the adhesive below. Grout alone cannot stop water from passing through to the tile adhesive, though. The grout must be sealed to prevent the passage of water.
Since unsealed grout is a porous material, when a grout line gets wet, the water is absorbed all the way through the grout where it can reach the tile adhesive. When tile adhesive gets wet, the adhesive breaks down, which leads to a sagging tile job. Eventually, the tiles may even fall from the walls. Sealing grout prevents the grout from absorbing both direct water and lingering humidity, protecting the integrity of the tile job.
Designed specifically for use on grout in showers and other areas of high humidity, silicone sealer comes in liquid form in a bottle. Generally, the silicone sealer container has a cap with a built-in applicator that fits the grout spaces almost exactly, making the sealer easy to apply, though some sealers may also come as sprays or in bottles without a built-in applicator. Silicone sealer dries clear to allow the grout color to show through.
Polyurethane sealer has many similar properties to silicone sealer. Like silicone, polyurethane applies over the top of grout in a nearly transparent layer that dries to a barely detectable coating on the surface of the grout. However, polyurethane is typically used in a larger variety of applications than silicone, while silicone is most commonly used in shower and other plumbed areas. So, polyurethane usually comes in a can or a bottle without a built-in applicator.
Before applying any type of grout sealer to a shower, make sure the surface of the grout is completely dry or moisture can get trapped beneath the sealer, leading to a murky discoloration over the grout. If the grout is dry and clean, apply the sealer using the built-in applicator brush on a grout sealer bottle or with a paintbrush roughly the same size as the grout line.