Metadata injection is a feature within Pentaho Data Integration that allows a transformation to inject configuration values into another transformation. This allows you to make reusable ETL patterns.
For example:
Imagine you have a transform that reads from a CSV file and outputs to a database table. Call this transform1.ktr
You could have another transform, call it transform2.ktr which knows the header column names and data types and formats. It could use those and using the Metadata injection step and target transform1.ktr and the Text File Input step as well as the Table Output step 'injecting' those settings into those steps; then cause the transform to execute.
ETL Metadata Injection - Pentaho Documentation
Steps Supporting Metadata Injection - Pentaho Documentation