The grout lines between your ceramic tiles aren't as hard and as waterproof as the tile itself. When moisture, dirt and grime get on tile, the grout is the part that's going to absorb the worst beating. If the tile was properly installed, the grout will have received a coating of liquid sealer to give it more protection. On a dry floor, these sealers are generally effective for two or three years before they need to be resealed, but on a tiled shower stall or sink backsplash you should do it once a year or even more often.
The surest way to test whether your grout needs resealing is to put a few drops of water on the dry grout and see if it beads up and sits on the grout surface, or soaks right in. If it's absorbed and disappears, it's time to reseal. For vertical tile, as in shower stalls, do the test by pressing one fingertip against the grout line and using an eyedropper to set the drop of water right on the fingertip, against the grout.
If your grout needs to be resealed, it first needs to be cleaned as thoroughly as possible so you're not sealing in dirt and grime. Use an old toothbrush and a little abrasive cleanser with water and completely scrub the grout lines. Rinse completely, making sure to get all the cleanser off the tile. Let it dry completely before applying the sealer.
Water sealer, like tile surfaces, comes in flat, semigloss and gloss finishes. Choose one that matches the finish of your tile. The easiest method is to buy sealer in a bottle with a small sponge applicator already attached. You just hold the applicator on the grout line, gently squeeze the bottle, and move the sponge down the line. Immediately use the corner of a damp rag to wipe away any sealer that gets on the face of the tile.