Drill Up/Down is sometimes also referred to as Slicing and Dicing too.
Basically, you have Hierarchies for your Descriptive information or Dimensions (like Time, Customers, Products, etc..), that aggregate the measureable information, facts (like sales amounts, quantities,etc..), naturally, based on the relationships between members the levels.
So, as an example, you might have a report that lists sales of Products by Year. You decide you want to see the sales of those products for a given Year's Quarters, so you would "Drill Down" on that year and see the 4 Quarters that comprise the year, where you formerly saw all the Years.
The Time dimension would be constructed with levels that represent the natural order of the time hierarchy
All Time
Year
Quarter
Month
Week
Day
Members of Year would be: 2006, 2007, 2008, etc...
Members of Quarter would be 2006 Q1, 2006 Q2, ... 2008 Q4, ...
This hierarchical structure is either defined in an OLAP data Source (such as Power Cubes, Essbase, SAP BW.... or in Framework Models, using DMR. (This is NOT available in ReportNet, only Cognos

HTH.... M