Summaries create aggregated totals in order to provide the user with higher level data as part of a more detailed report. There are several types of summary that can be included in a table:
Sub Totals are used when there are multiple records per category, possibly caused by having multiple dimension fields in a table, and it would be beneficial to view a total value for each category value. The example shown here could use a sub total for Year, as there are multiple rows per year value due to the Quarter field.
When using Sub Totals, we often pair this functionality with the Suppress Duplicates option, to make the report output easier to consume. To apply this option:
Totals and subtotals can be moved to the start of a table or section too, so totals might appear in the top row and left column and subtotals might appear at the top row or column of their section within a table. In the Column Formatting dialog box, look for the Move Total Value Location toggle (and the Move Sub Total Value Location toggle, further down) and activate them to see totals at thes tart of a table and subtotals at the start of a section.