Drill Down and Drill Up (also known as Data Drilling) means to navigate in hierarchical dimensions of data stored in Data Warehouses. There are two opposite ways of data drilling:

• Drill Down is used within Online Analytical Processing (OLAP) to zoom in to more detailed data by Changing Dimensions
• Drill Up means to zoom out stepwise (also known as roll up)

If you have catalogue data about used cars stored in a database, first dimension could be the type of cars (limousine, SUV, etc.). Second dimension could be the manufacturer, third the production year. So you can drill down from type over manufacturer to year of production, reducing the amount of found data in every step.