The "Placeholder Source" servers two purposes as part of Task Factory.
Use as a Data Generator
The "Placeholder Source" can be used to generate random data based on the columns defined in the "Placeholder Source". In this option, you define the columns and the number of rows you would like generated randomly by setting the "Number of Rows to Generate" option.
Use as the stat of a Data Flow Nugget
The "Placeholder Source" is used to mark the beginning of a reusable Data Flow Nugget. When used as the beginning of a "Data Flow Nugget" the number of rows to generate option is ignored.