Complex sentence: A'Polish joke' belongs to the category of jokes whose understanding requires prior knowledge of "what a Polish joke is", because similar to all discriminatory jokes, they depend upon the audience's preconceived notions and "affective" dislikes for entertainment.
Split into simpler sentences: A'Polish joke' belongs to the category of conditional jokes, i.e., jokes whose understanding requires from the audience a prior knowledge of "what a Polish joke is". Similar to all discriminatory jokes, they depend upon the audience's preconceived notions and "affective" dislikes for entertainment.