A pivot table summarises a list — totals by category, counts by month — without writing a formula. It takes about thirty seconds once the data is shaped right.
Steps
Make sure your data is a plain rectangle: one header row, no blank rows, no merged cells, one thing per column.
Click any cell inside it, then Insert → PivotTable. Accept the range it guesses and choose a new worksheet.
Drag the field you want to group by into Rows — the category, the region, the month.
Drag the number you want summarised into Values. Excel sums numbers and counts text.
To change how it summarises, click the field in Values → Value Field Settings → Sum, Average, Count.
Drag a second field into Columns to cross-tabulate, and into Filters to slice the whole table.
Worth knowing
If a number column is being counted instead of summed, there is text somewhere in it — often a stray "n/a". Refreshing after the source data changes is manual: right-click the table → Refresh. Turning the source into a Table first (Insert → Table) means new rows are picked up automatically.
Faster than searching: box the part of your screen you
cannot work out and ask about it directly — in Excel or anywhere
else. Answers arrive where you drew the box.