The word Mindfulness is the combination of words "Mind" and "Fullness", which means Completeness of the Mind. So, Mindfulness is using your "Complete Mind" in the present moment without any judgments. Mindfulness is not excessively active or overthinking about what is going on around us, it is a simple way of living and observing the present without any judgments. Some people understand that Mindfulness is only being in present, that is not the complete truth. Yes, it is the truth but not complete.
Let's understand the complete truth about Mindfulness. For example, you are making a beautiful coffee for yourself and you are being mindful of it. You have done everything perfectly and now a beautiful and tasty coffee is ready. You are taking your coffee out and going towards the Garden to enjoy it. Suddenly, the cup slipped from hand and coffee dropped down on the garden. Now you are angry because you did everything perfectly but the results are not perfect.
So, you might be thinking that if I follow Mindfulness, so why I am angry? Actually, you have followed only the first step of Mindfulness. So whatever Karma we do, we get back reaction of it, Action always has reactions back. So if you make coffee, it will give you reactions back but if your shoe hits the stone before reaching the garden, the coffee cup will definitely slip from hand. But here you have not to Judge results because results are never in our hands.