Meaning of $\stackrel{*}{\rightarrow}$ production rule?
It's either the transitive closure or reflexive-transitive closure of $\rightarrow$. That is, $A\rightarrow B$ means that you get from $A$ to&nbsp;$B$ in exactly one step; $A\rightarrow^*B$ means either you get to $B$ in one or more steps (transitive closure) or in zero or more steps (reflexive-transitive closure).