Lamer (or grigner) is a baker's gesture that consists in slitting the top of a loaf of bread or viennoiserie with a razor-sharp blade, just before putting it in the oven, so that it can rise better during baking.
The stroke of the blade creates a weakness in the surface of the dough, a weakness that will lead to cracks and openings as the dough rises during baking.
Note that if you don't blade a dough, it tends to burst or tear during baking.