At Duolingo's headquarters, they have a huge aquarium full of highly trained manatees. On a far end, they have heavy balls with words printed on them. They're called 'idea balls'. Manatees pick those balls up, and drop them at the sentence generator input hole, which is on the other end of the pool. Since several manatees work simultaneously, the balls have usually different words on them, and sometimes weird sentences turn up. But those manatees do not like it when the moderators at Duolingo reject some sentences based on how much sense they make. So they show their protest by halting idea generation. You see, manatees are extremely ethical and extremely intelligent creatures. To them, either everything makes sense, or nothing does. I hope this helps.
Yes "du" means an uncertain amount.
"Je mange du riz" = "I am eating some rice"
But "some" is optional in this English sentence so we can leave it out without changing the meaning of the sentence.
So "Je mange du riz" also translates as "I am eating rice"
The important thing to remember is that in French "du" is not optional. We can not leave it our.