A variable $A$ in a context free grammar $G= \langle V, \Sigma, S, P\rangle$ is live if $A \Rightarrow^* x$ for some $x \in \Sigma^*$. Give a recursive algorithm for finding all live variables in a certain context free grammar.
I don't necessarily need an answer to this. Mostly I am having a very difficult time deciphering what this question is asking. More specifically its definition of live variables.