Most often, the reason something is "on your mind" is that you want it to be different than it currently is, and yet: you haven't clarified exactly what the intended outcomes is; you haven't decided what the very next physical action step is; and/or you haven't put reminders of the outcome and the action required in a system you trust. That's why it's on your mind.
Chapter 1 (Getting Things Done: The Art of Stress-Free Productivity (2001))
First of all, if it's on your mind, your mind isn't clear. Anything you consider unfinished in any way must be captured in a trusted system outside your mind, or what I call a collection bucket, that you know you'll come back to regularly and sort through.David Allen
Here's how I define "stuff": anything you have allowed into your psychological or physical world that doesn't belong where it is, but for which you haven't yet determined the desired outcome and the next action step.David Allen