Fills empty rows with the row above's value
If you work with data that comes out of a system that is remotely complex or performs sophisticated data analysis, then you have seen tables with columns on the left that are incomplete. The column will have a value, followed by a series of blanks below it until you hit another value. This is called Hierarchical or Multi-level indexing, and is quite useful when condensing complex data.
The problem is that most tools that the average data user has at their disposal do not handle Multi-level indexing well or at all. Usually, you just need the data to be in a complete table format. The Fill Down step will target a column and fill its values in from top to bottom, using the value from the previous cell.