What does "All store and access operations (for S(t) , A(t), and R(t)) can take their index mod n + 1" mean?

It's from the book Introduction to Reinforcement Learning. Second edition, chapter7: n-step Bootstrapping, page 147, n-step Sarsa. I made the algo work, but I still don't understand the phrase. Preferably explained in Python terms. The introductory part for the algo: Initialize Q(s, a) arbitrarily, for all s $\in$ S, a $\in$ A Initialize $\pi$ to be $\epsilon$ -greedy with respect to Q, or to a fixed given policy Algorithm parameters: step size $\alpha$ $\in$ (0, 1], small $\epsilon$ > 0, a positive integer n All store and access operations (for S(t), A(t), and R(t)) can take their index mod n + 1