The phrase "undecided voters" refers to people who have not yet made up their mind about which candidate or option to choose in an election or a decision. They are unsure and still need more information before making a final choice. Full definition
Candidates would need to care about voters across the nation, not just undecided voters in a handful of swing states. (nystateofpolitics.com)
That said, with the polls as close as they currently are it's worth remembering that undecided voters who finally make up their minds could have a major impact on the outcome. (totalpolitics.com)
We watched undecided voters as they watched the debate hoping it might help them make up their minds. (itv.com)