If you've got a tiled backsplash, tub surround or shower stall, there is grout between the tiles. Depending on how the tile was installed, there may also be grout in the spaces around the perimeter of the tiled area, where it meets the tub, countertop or shower floor. Those areas should be caulked rather than grouted because they're susceptible to cracking from movement between the materials. However, if those lines are already grouted, you can caulk right over them.
Dip an old toothbrush in bleach and scrub all the grout lines in the corners around the perimeter of the tiled area. Get the lines free of all dirt and loose material. Let it dry for 24 hours.
Set a caulk tube in your caulk gun. Cut off the end of the tube at a diagonal angle, making a hole about ¼ inch wide. Shove a long nail into the hole to open the seal inside the tip.
Set the tip of the tube against the end of the first grouted line that you scrubbed. Position the gun at a 45-degree angle. Compress the trigger until the caulk comes out the end.
Move the gun backward along the grout line, continuing to compress the trigger, expelling a straight bead of caulk all the way to the end of the line.
Wet your thumb. Hold the ball of your thumb against one end of the new caulk line and smooth along the length of it, slightly flattening it and getting it to look clean and finished. Repeat the process for each grout line around the edges of the area.