What is the column to the left of sheet

Some sheet have a column to the left of the sheet. Some of these columns have a 1/2 heading and a +/- next to a row. Clicking the +/- hides or displays the row.

Hallo

→Data→Group & outline→Group|Ungroup

Looks you are comment about grouping rows/columns

https://help.libreoffice.org/Calc/Group

Documentation Calc guide
Calc Guide: Full book - page 71