Mindfulness is a big word, closely associated with some (usually pretty simple) meditation practices. Overall, it does what is says on the tin, you are more aware of the stuff going on in your mind.

Try to sit down for 10 minutes (set an alarm) and actively notice all the thoughts going through your head. Try to quite the internal dialogue, try to prevent words from going through your head.

Just this exercise usually turns people on to the existence and overall velocity and erratic nature of your mind's ongoing conversations, stimuli, reactions to stimuli, etc.

If you've ever gone on a successful campaign starting at couch potato and ending up in a place where you miss your 45m of exercise if you don't do it, there's a similarity.