Select the 
Add Comment item in the cell's context menu.
Press SHIFT + F2.
To add explanations, reminders, comments for sheet cell, use comments.
Comments can be multiline. The cell with a comment is marked with a red triangle at the top right corner. To view a comment, hover the cursor to the cell.
To add a comment for the selected cell, execute one of the operations:
Select the Add Comment item in the cell's context menu.
Press SHIFT + F2.
The Comment dialog box opens:
The following operations are available when working with comments:
See also:
Edit comment
Clear button on the Data