What does the〈 〉notation mean in this situation?
From the context of the exercise, it appears that $\langle x\rangle$ is meant to denote a canonical representation of the integer $x$ as a bitstring of length $n$.

If this were not the case, those inputs and outputs would not be in the correct  domain and range respectively.

Likely this notation was defined somewhere in the lecture notes, but I cannot find them at the moment.