What is the definition of the following statement?
The rewriting rule $l_{1}\rightarrow r_{1}$ is an instance of another rule $l_{2}\rightarrow r_{2}$.
PS:This statement comes from the paper of term rewriting systems which is called "only prime superpositions need be considered in the Knuth-Bendix completion procedure".
As already explained in the comments, an instance $\ell_1 \to r_1$ of a rewrite rule $\ell_2 \to r_2$ is a rewrite rule that is obtained by applying a substitution to $\ell_2 \to r_2$, thereby instantiating variables in $\ell_2$ and $r_2$.