# what does “Configuration is valid for a viable prefix ” actually means?

I am learning LR(1) parsing from the Dragon Book. I am really confused with this sentence.

Formally we say that a configuration $[A \to u•v, a]$ is valid for a viable prefix $α$ if there is a rightmost derivation $S \Rightarrow^* βAw \Rightarrow^* βuvw$ where $α = βu$ and either $a$ is the first symbol of $w$ or $w$ is $∂$ and $a$ is $\$\$.

I am not able to figure out how validation of a configuration for a viable prefix is useful . What does it speak intutively and why does it matters?