Some daily-life examples to help explain what 'mindfulness' and 'mindless activity' are?
When your Buddhism teacher tells you: "don't get irritated in daily life, don't get frustrated, be the master of your mind"

-- and then you come back next day and say, "I really wanted to be a master of my mind... but then I got carried away and yelled at my wife"

and teacher says: "but this was supposed to be your main practice, to not get irritated, didn't I give you instructions?"

and you say: "somehow, in real life, when we started arguing with my wife, it looked so real and so important, and your instructions seemed so far away and so naive and irrelevant"

and the teacher says: "this means you lack mindfulness. You must hold your practice assignment in front of you at all times, it must be the main thing on your mind - whatever else you do in daily life. Develop mindfulness. Do not get carried away."



So that moment when "it looked so real and so important, and your instructions seemed so far away and so naive and irrelevant" - is when your perspective (and with it, your perception of reality and hence your judgement) has shifted by the external circumstances. If you had mindfulness, you would be able to hold your perspective the way your teacher has set it up, regardless of external pressure. So mindfulness is really a basic form of jhana, being able to control your perspective, instead of you being a slave of perspective. Mindfulness is predecessor to meditation.