Transform > Group By collapses rows that share a value into one row, and works out something about each group: Count Rows, Sum, Average, Min, Max or Count Distinct Rows of a column.
Basic gives one result column; Advanced lets you add several aggregations, and group by more than one column. The output has the group columns and the new result columns only; every other column is dropped, which is what a summary table should look like.
Choose the operation with care: Count Rows counts orders, Sum adds up the values of a column. Mixing them up gives believable but wrong numbers.