The Simple Mapping transform allows you to re-use a series of transforms in the form of a "re-usable pipeline" also known as a "Mapping".
Use this transform if you find yourself repeating the same logic multiple times over several different pipelines.
The options are fairly self-explanatory:
Transform name: a unique name in your pipeline
Pipeline: The name of the mapping (sub-) pipeline to embed. This pipeline is executed separately and is considered a separate entity. It is always executed with a standard local Hop pipeline run configuration since that is currently the only one capable of streaming data into and out of transforms.
Parameters tab: specify the parameters you want to pass to the mapping pipeline
Input tab: specify which fields to pass to the Mapping Input transform in the mapping. You can map (hence the name) field names.
Output tab: specify how to rename output fields retrieved from the Mapping Output transform.