If your mom says, "What have you been up to?" and you reply, "Ah, stuff," you have used a generality. It's the opposite of being specific, of saying, "Well, I shop lifted and picked my nose."
Politicians speak in generalities––they want to cut the deficit and keep taxes low, but they don't want to tell you exactly how they will do that. Generalities can also refer to conclusions we draw about the world: If you say people at baseball games like to eat hot dogs, you've used a generality. Maybe people at baseball games eat hot dogs because there's nothing else to buy.