I am learning LR(1) parsing from the Dragon Book. I am really confused with this sentence.Formally we say that a configuration $[A \to uv, a]$ is valid for a viable prefix $$ if  there is a rightmost derivation $S \Rightarrow^* Aw \Rightarrow^* uvw$ where $ = u$ and either $a$ is the first symbol of $w$ or $w$ is $$ 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?
what does Configuration is valid for a viable prefix  actually means?
terminology;compilers;parsers;parsing