What do the words “Does not contain kitniyot” that appear on the box of sweetened mud marketed as kosher -for-Pesach brownies have to do with Pesach? 

Kitniyot are pulses or beans in Hebrew.  The word kitniyot derives from katan, meaning small, and appears in the Talmud.  

Medieval Ashkenazic halachic decisors ruled that kitniyot were forbidden on Pesach. Some argued that since the sacks used for harvesting beans were the same as those used for wheat, a little wheat could accidentally get mixed in with the beans. You could then innocently cook wheat with your bean stew on Pesach and serve your family chametz.

Another less known reason for the ban on kitniyot is that many rabbinic leaders saw the potential for beans to be ground into flour and baked into cakes as problematic. If people were allowed to eat cakes on Pesach, it could lead to confusion.  

It can be a shock for Ashkenazi diaspora Jews to find that most of the foods sold in Israel over Pesach contain kitniyot, as Sephardic Jewry never adopted the ban. Something that for centuries has been forbidden for one community is eaten freely by another. Some Ashkenazi olim say that it is time to abandon such communal peculiarities in favour of the unity of the nation.  But apart from the halachic issue, one could argue that these minor differences enrich our culture — pass the potatoes, please.