Help screen Column Filtering Sort Number Description
From Joomla! Documentation
These column sorting input fields shows the current method of sorting applied to the table. Use the dropdown field choices and click the column heading name. Alternatively, click on the column heading to sort the list by that column's value.
This will also change the value in the drop field to correspond with the column header name. The list will then be sorted in order by that column and a sort icon (up or down arrow) will show next to the column name. Click a second time to reverse the sort, which will change the corresponding drop field.