A MediaItem is nothing without it's conversions. A conversion is the content of the MediaItem in some form. It can be the original content, a scaled image or video, a conversion to a PDF or some other conversion.
A conversion retrieved by specifying which MediaFormat that will be used. Not all media can be converted to all formats. This depends on the installed converters. If a conversion cannot be done, a null result will be returned when retrieving the specific conversion.