While filtering allows you to choose several entries that meet the predefined requirement, grouping helps to divide all data on collections each of which has something common. To create a group of data, click 
icon from the toolbar (1), add fields used to group by (2), define their order (3) is there are more than one field chosen and click OK (4):
Using the conditions above, the Library data will be divided into groups by Category. Inside this group, the entries will be grouped again, if any of them have the same Quantity:
Note that all aggregations added to this Library will be calculated for each group.
To remove grouping, click icon from the toolbar, choose all fields in Group order list, double-click them and click OK.