Transmogrifier provides support for building pipelines that turn one thing into another. Specifically, transmogrifier pipelines are used to convert and import legacy content into a Plone site. It provides the tools to construct pipelines from multiple sections, where each section processes the data flowing through the pipe.
A "transmogrifier pipeline" refers to a description of a set of pipe sections, slotted together in a set order. The stated goal is for these sections to transform data and ultimately add content to a Plone site based on this data. Sections deal with tasks such as sourcing the data (from textfiles, databases, etc.) and characterset conversion, through to determining portal type, location and workflow state.
Note that a transmogrifier pipeline can be used to process any number of things, and is not specific to Plone content import. However, it's original intent is to provide a pluggable way to import legacy content.