To label collapsed columns in Excel, follow these steps:

1. Select the cell next to the "+" sign of the collapsed column where you want to add the label.
2. Right-click on the cell and select "Insert Comment" from the drop-down menu.
3. Type in your label text in the comment box that appears.
4. Click outside the comment box to close it.
5. Move the comment box to the desired location by clicking and dragging the border of the box.
6. Resize the comment box by clicking and dragging the corners of the box.

Now, when someone hovers over the cell with the comment, they will see the label text. This is a great way to provide additional information about the collapsed columns without cluttering up the worksheet.