A plebiscite is like a referendum - where all voting citizens are allowed to voice their opinion on an issue, by voting at a ballot box on what they think the government should do. It's a lot like an election, except that you're voting for an 'idea', instead of for a person.
The only difference between a referendum and a plebiscite is that it isn't binding on the government. Instead, the results of a plebiscite are considered as 'strong advice' to the government, and should be used to guide decision-making on the issue in question.
A win for PR in this November's plebiscite would mean that the government will have no choice but to replace our broken “First-Past-The Post’ - it just wouldn't be right to ignore the will of the people!