Fodder literally means food for livestock. Figuratively, it usually refers to random unnamed mooks or soldiers that are in plentiful supply but have no skills to speak of. In fiction, these kinds of characters usually get stomped all over by any kind of named fighter character. In the OBD feats of defeating fodder characters are generally not looked highly upon, given the ease with which any fodder can be defeated. Still, fodder from one verse can be so high and above even main characters from another verse that defeating fodder in the former verse easily allows for that character to be a threat in the latter verse. Also, named characters with no feats or reasons to be powerscaled can be considered fodder-level.
Examples of fodder:
Stormtroopers (Star Wars)
ANBU Black Ops (Naruto)
Firebender Troopers (Avatar: The Last Airbender)
Red Shirts (Star Trek)