I have a spreadsheet with several simple worksheets.

In one of the worksheets, at the very left, there has appeared a small "1
2".  Under that is a vertical line and a series of dots, one dot per row. At
the end is a minus sign.

If I click the "1", most of the spreadsheet disappears, showing only the
final total column with a plus sign on the left. If I click "2", the full
spreadsheet reappears.

I've tried searching Help to find out what this is and how to get rid of it,
but as I don't know it's name, function, purpose or how it got there, I'm
totally lost!

Please tell me what this is and how I can remove it.

(Microsoft Excel 2003)

Thank you