What is this mysterious side-column in Excel?

I've never seen this side column in an Excel worksheet before. What is it called?

Clicking on the '2','3', and '-' buttons seem to show/hide rows 3-4; clicking the '1' button seems to do nothing.

Solution 1:

It is data grouping / outlining.

Check the Data tab/menu, specifically the "Outline" commands, group, ungroup, and subtotal.

Search help for "group" or "outline"

Outline (group) data in a worksheet at http://office.microsoft.com/client/helppreview14.aspx?AssetId=HA010342744&lcid=1033&NS=EXCEL&Version=14&tl=2&respos=0&CTT=1&queryid=17b08283%2Dd961%2D4c62%2D9372%2D4a1de31c4b4f

Solution 2:

This is the "outlining" feature of Excel. It allows you to group rows together so they can be expanded/collapsed easily. It is useful if you're making a sheet that has heirarchical data in it. I'm not sure about Excel 2010, but in earlier versions, you can access the feature via Data -> Group and Outline.