The im_convert function originated within the Oracle interMedia group (now called Oracle Text). interMedia's query language is not something we should expect end users to learn. interMedia's response was to use this function to translate a human entered query into a string that interMedia could understand.
If you're interested in understanding what each of the operators does a good start is http://www.arsdigita.com/asj/intermedia/intermedia.html
Greenspun has a paragraph about im_convert at http://philip.greenspun.com/internet-application-workbook/examples-search/site-wide-search