Culls are basically "reject" fish.. the runts, the ones with deformities... culling is the process of killing off fish due to problems with the batch.
Basically, a breeder euthanizes any fish that can't develop properly or fend for itself without special treatment. Some -good- breeders don't euthanize the culls, and try to give them the best life possible for them.
If you go to the deformities thread, there is an example of an open mouthed cull. That is a reason some breeders will cull, and cull early.
It tends to be unpleasant and people get riled up over the process of culling.