Most people lack the practice and also the instructions to be truly mindful. It's one thing to read about it, but an entirely different matter to do it. Mindfulness is the essence of understanding, because without it there is no seeing into the heart of any phenomena. Mindfulness is not judgmental, nor discriminating, nor telling stories. Mindfulness knows when there is concentration and when there isn't when the mind wanders off and when the mind becomes peaceful.
Perfect mindfulness knows every moment that is occurring.
If we're not paying attention, we're not really awake. We need to practice clear attention to any one of these at all times. It is possible that in meditation the mind becomes concentrated. If there is a feeling of peacefulness, one has to know that quite clearly. Without realizing what is happening, one cannot go further, because one doesn't know where one is at.