What is $\prod_{n+1}(x)$ in the inequality $|R_n(x)|\le \frac{M_{n+1}}{(n+1)!}|\prod_{n+1}(x)|?$

Question

What is $\prod_{n+1}(x)$ in the inequality $$|R_n(x)|\le \frac{M_{n+1}}{(n+1)!}|\prod_{n+1}(x)|?$$

$M_{n+1}$ is defined as $M_{n+1}=\max_{t\in[a,b]}|f^{n+1}(t)|$ where $a=\min\{x,x_0\},b=\max\{x,x_n\}$. What is the definition of $\prod_{n+1}(x)$?