automatically creates hierarchical cell groups on the basis of cell style.
Details
Automatic Grouping organizes cells hierarchically based on the styles of the cells in a notebook. For example, Input cells form groups automatically only when followed by Output cells.
The grouping behavior of cells is controlled by the CellGroupingRules option.
Any groups you have created manually will be ignored when Automatic Grouping is selected.
Automatic Grouping sets the option CellGrouping->Automatic on the selected notebook.