Charring technically means partially burning or blackening part of the surface. We do not want to burn the tortillas obviously but we want to get a good char on them. Using a cast iron skillet is best when you want to char foods and adding oil to the food directly that you want to char helps as well. You can also place the tortillas directly on the burner of a gas stove until you get that nice blackened char. This give a smokey flavor with a little bit of crunch.