The drill up and drill down function in Power BI allows you to see data in multi-levels. However, drill requires data hiearchy., which refers to the systematic organization of data that helps to show the relationships between smaller and larger components in a database. An example of data hierarchy from largest component to smallest is business unit - department - teams - employee. 

Drill Pathways 

In Power BI, there are four drill pathways available: three drill down pathways and one drill up pathway. 

• Drill up pathway: 
  • Drill up one field 
• Drill down pathways:
  • Drill down one field at a time 
  • Drill down all fields at once
  • Expand all fields at once 

Drill up one field 

The drill up icon allows you to go back up to the previous hiearchy, 

Drill down one field at a time

Click on this icon to turn on the function. Now, you have the option to drill down one field at a time by selecting a visual element. Examples of visual elements are bar, bubble, each row of a table, etc. If you don't turn on the drill-down icon, selecting a visual element won't drill down but cross-filter the other charts on the report page. 

Drill down all fields at once

Selecting this icon will take you to the next level in the hierarchy. 

Expand all fields at once 

Expand adds an additional hierarchy level to the current view. Each step in the path shows you the same information and adds on one level of new information