Student or Learner
As I read it, it means that he's putting the sugar back in the jar with something extra (dust and whatnot from the floor). It's like talking of the interest rate a bank/account gives you: you deposit your money there, and when you take it back, it is (usually) a bit more than you had. So you have your money back with interest.
Please be aware that I'm neither a native English speaker nor a teacher.