The set of prefixes of right sentential forms that can appear on the stack of a shift-reduce parser are called viable prefixes. An equivalent definition of a viable prefixis that it is a prefix of right sentential form that does not continue past the right end of the rightmost handle of that sentential form
Link: https://en.wikipedia.org/wiki/Viable_prefix
read this @ankit
https://stackoverflow.com/questions/4202181/explanation-about-viable-prefix
@Bikram sir confirm this,