A data mining and spreadsheet term.
It is the ability to click on a piece of data to further expand on that information. You 'drill down' the data.
For example a cell in a pivot in Excel might have "1020" as the total sales in a shop. Double clicking on the "1020" may cause another worksheet to be produced showing the individual sales that made up the total. This is 'drill down'.
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Click on this link: Drill Down