Cell Comments-When the office assistant is turned on the comment

I have inherited a spreadsheet but don't know how to edit it. When clicking on certain cells a comment appears - but it is not a normal comment as there is no red triangle in the corner of the cell.

In the edit/clear menu I have the following options - all/formats/contents/columns. Only the first removes the comment. Can't find any other menu which effects it and I can't double click on it.

When the office assistant is turned on the comment appears as a speech bubble.

What is this comment (which is not a comment) and how do I edit it?