Bread scoring means cutting the surface of shaped, proofed dough just before baking. The cuts provide places for the loaf to open as it expands, helping control the shape of the crust. A sharp blade or bread lame makes a clean cut without dragging.
Cut depth and angle depend on the loaf and design. Scoring can direct oven spring, but it cannot repair a severely underproofed or overproofed dough.
Reference: Further reading.