Slashing the dough before baking does more than 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 under the crust? Well, slashing your loaf helps prevent this.
One long slash down the center is a nice finishing touch for a sandwich loaf. 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.
Before slashing, 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 want. You can also leave the bread plain.