Inserts a comment at the currently selected cell(s). Comments are electronic 'post-it' notes attached to cells.  They contain the name of the author by default and depending on the setting under Tools>Options...>View, will either, show as a small red indicator triangle in the upper right of the cell (default), the indicator and comment will show or nothing will show at all. If you have the default 'indicator' showing, the full comment will be shown as you move the mouse over the cell that contains it. Comments can be edited by either returning to this menu option when a commented cell is selected, (the option will have changed to Edit Comment) or right clicking on the cell and selecting Edit Comment. Comment boxes can be re-sized by dragging the 'sizing handles' (hold down the Alt key to size them exactly to cell heights and widths) and formatted with regard to text colour, font and style, (as below).

 Comments can be printed in-situ or at the end of a printout, by selecting the options in File>Page Set-up...>Sheet tab. Equally, I have some code which prints all comments in a workbook to a text file with the source address and comment and also copies comments across sheets.