What does "rub in" mean?

Also, rub it in. Harp on something, especially an unpleasant matter, as in She always rubs in the fact that she graduated with honors and I didn't, or I know I forgot your birthday, but don't keep rubbing it in. This idiom alludes to the expression rub salt into a wound, an action that makes the wound more painful; it dates from medieval times and remains current. [Mid-1800s] Also see rub someone's nose in it.