From Being Dharma: The Essence of the Buddha's Teachings, by Ajahn Chah:
In practice, some come to see easily, some with difficulty. But whatever the case, never mind. Difficult or easy, the Buddha said not to be heedless. Just that--don't be heedless. Why? Because life is not certain. Wherever we start to think that things are certain, uncertainty is lurking right there. Heedlessness is just holding things as certain. It is grasping at certainty where there is no certainty and looking for truth in things that are not true. Be careful! They are likely to bite you sometime in the future!
What is heedlessness? Not only is it not paying attention, but it's not caring to pay attention. When we're heedless, we do out of habit, not out of conscious deliberation and choice. But it goes deeper than just doing out of habit. It strikes right at the heart of your inner mind. When we're heedless, we don't CARE enough to pay attention. If we did, we'd be paying attention and choosing our actions consciously.
When a loved one asks, "Did you pick up the detergent from the market?" And you answer, "No, I forgot." Did you? Sometimes you really did, and in that case, there is no problem. But often you remembered, but really didn't feel like going. Maybe you made a conscious choice not to go; or maybe you "allowed" something to distract you so that you were sure to forget.
But then you get home and answer, "I forgot." That's an easier answer, isn't it? But it's heedless. Usually such an answer just flows from your lips. That's habitforce, and it's proof that you just don't care enough--that you truly don't comprehend the benefit of--paying full attention and seeing the negative effects of your lack of mindfulness in allowing yourself to lie. Maybe you did consciously choose to fib in your answer; maybe that took some careful deliberation to decide. But it's still heedless because you chose to ignore the negative effects of such a lie.
1-Minute Meditation: Where have you been heedless today? Where have you allowed yourself to act under force of habit, as though all things were certain and not requiring of consideration, including potentially un-thought-of effects? Now look deeper. Why don't you think it's important enough to be heedful? [Ego might answer, "I *do* care!" But be honest. If you really did care, you would have been heedful in that moment, wouldn't you?]