(whom or what)
This seems to be a common way for Polish people to describe cases. I'm not convinced it's a useful method for English speakers.
This is how we are taught at school... each case is a response to a specific set of questions...
It was almost like the multiplication table:
Mianownik: Kto? Co?
Dopełniacz: Kogo? Czego?
Celownik: Komu? Czemu?.... (i tak dalej)
However this is for Polish speaker learning Polish grammar...