Help with popup box when clicking on cell, can't remember how to create popup box

I've added a box that pops up when I click on a cell which visually reminds me how to input information into the cell ... my question, did this back 4 or 5 years ago and can't remember how to create again for a new cell (hope I'm clear on this) ... don't even remember what the pop up box is called, I know its not a comment box ... see RED box in attached image - Thanks in advance to your help