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