Excel: How to show a floating text box only when I select the cell?

There’s a type of text box in an Excel file that is associated with a cell
and appears only when you select that cell uniquely
(e.g., by clicking on it or moving to it with cursor keys). 
Hovering over it doesn’t do it; selecting it as part of a range doesn’t do it. 
It’s somehow like a comment, only that cells with comments show a little red mark on the upper-right corner of the cell,
and the comments appear when you hover over the cell.

What the worksheet normally looks like: no text box appearing:

What it looks like when the cell is selected: text box appears:

What are these text boxes called, and how do I make them?