A mental note is to make a particular effort to pay attention to something so that you will remember it later

Exactly what Todd said! For example, in class you would take notes (on paper) of the most important points. A mental note is essentially that, but (instead of paper) you are just remembering that particular thing. Like Todd said, you pay particular attention to remembering it.

Ok, here are my two cents:

imagine you're holding a piece of paper, you jot down something that must be remembered, needs to be finished or can't be postponed. Everytime you're about to forget about that thing all you need to do is look at this piece of paper in your mind.

A mental note is in some way a short-time memorization activity.