Resource name?

Greetings,

I recently downloaded a sample workbook and, while analyzing its structure, I noticed a panel between the formula bar and the column index, as shown in the image:

Excel.png

Clicking on the square labeled 1 will hide columns A to O (which have the black line spanning atop them), while clicking the square 2 will show everything.

My question is, what is the specific name of this resource and how can I add / edit it?

Thanks in advance

Thats it, thank you very much!