Slashing the dough before baking does more than just make a pretty pattern on your bread. It also helps direct how the bread will rise in the oven. Have you ever baked a loaf of bread and ended up with a large air pocket right under the crust? Well, slashing your loaf helps prevent this.
For a sandwich loaf, one long slash down the center of the loaf is a nice finishing touch. If you are baking a round loaf, a "Tic Tac Toe board" slash will let your bread rise evenly all the way around. For long, slender loaves like baguettes, a series of angled parallel slashes down the length of the dough gives you a classic baguette-look.
Prior to slashing, you can brush the dough with egg wash, water, milk or egg white. Now is the time to add some poppy seeds, sesame seeds or whatever topping you would like. You can also leave the bread plain.