Inmod: It is a user written routine to pre-process the input data. It works with mload,fload, tpump and fast export. It is just an extension to the utility.
Outmod: It is a routine used for special output processing along with FastExport utility.
Inmod , outmode routines are object codes (executable ) of any third party language programs such as c, c++, cobal which would be used as pre or post processing correspondingly.
For example while you are loading the data from a file and in the file you have 100 columns and you want to load only few columns with some concatinations...etc..you can write a program in c, c++ languge implementing this functionality compile it and invoke the object code through fast load utility. This inmode will pre porcess the data and gives the output which in turn used by fast load to load into your target.
similar with outmode as a combination of fast export (after exporting data if any optation or validation you want to on output file)