What is Backward-looking rate, forward-looking term rate?

A backward-looking rate for a period $[S,E]$ is the overnight rate compounded in arrears over the period (One Quant Book 2, chapter 1): $1+\delta R(S,E) = \prod_i(1+r_id_i/360)$, known only at $E$. A forward-looking term rate for the same period is published at $S$, as an expectation of that compounded rate implied by futures and swaps (a term SOFR rate) or as an interbank offered rate (Euribor); it is known at the start.