Programming is difficult, and learning by its very nature is going to involve a lot of things which are “unknown”. A good amount of your time, as a programmer, will likely be spent looking for information and learning about things you have either a) not seen before and/or b) don’t fully understand.
An example would be the use of
mod in this context: you saw it, didn’t understand it, and went looking for more information to help you make sense of it before ending up here. This is a process with which you should become familiar.
I can only presume, as I do not have access to the specific exercise (since I am neither a Pro member nor a Codecademy employee), that since this exercise deals specifically with the modulo operator that the specific statement of
a mod b is intended to prompt the use of the
% operator you have been learning about. It also introduces you to the notation.
Otherwise, if the exercise simply asked for a remainder, you might derive some alternative mechanism for calculating it when a single operator is sufficient.