A reality check is something you do when you're awake, throughout the day, repeatedly, so that your brain gets into the habit of doing it, so that when you think you might be in a dream, you do it, and if the outcome isn't as it should be (you have six fingers instead of five) you know it's a dream, and therefore, can acknowledge the fact that you are in a dreamstate, thereby entering lucidity, thus allowing you to beging controlling said dream?