HoudahSpot uses the macOS Spotlight index for ultra-fast file searches. By building upon Spotlight, HoudahSpot can find files by name, text content, as well a wide variety of file properties and metadata.
Spotlight in turn relies on importer plug-ins to read all kinds of files. When indexing a file, Spotlight calls upon the plug-in that specializes in the file type at hand to extract text and metadata. Such importers are installed with the system and with applications that introduce new file formats.
The Info pane in HoudahSpot shows the information available for a given file. This is the metadata that the importer plug-in has shared with the Spotlight index.