Right-click the cell to which you want to add a comment, and select Insert Comment from the shortcut menu that appears.
Type the desired text into the comment area. When you're finished, click anywhere in the worksheet to accept the comment.
The cell's upper-right corner now contains a red triangle, indicating the presence of a comment.
To view the comment as a ScreenTip, move the mouse pointer over the triangle.
INTRODUCTION
Some cells contain data or formulas that require an explanation or special attention. Comments provide a way to attach this type of information to individual cells without cluttering the cells with extraneous information. A red triangle indicates that a cell contains a comment, which you can view in several ways.
TIP
Working with Comments
To quickly edit or delete a comment, right-click the cell that contains the comment marker and select the desired command (Edit Comment or Delete Comment) from the shortcut menu that appears.
TIP
Displaying Comments
You can make it so that Excel automatically displays the full text of a cell's comments while you work in the worksheet. To do so, right-click the commented cell and select Show/Hide Comment from the shortcut menu that appears.