Wikipedia
IFilter
An IFilter is a plugin that allows Microsoft's search engines to index various file formats (as documents, email attachments, database records, audio metadata etc.) so that they become searchable. Without an appropriate IFilter, contents of a file cannot be parsed and indexed by the search engine.
They can be obtained as standalone packages or bundled with certain software such as Adobe Reader, LibreOffice, Microsoft Office and OpenOffice.
It also refers to the software interface needed to implement such plugins.