Quadratic voting is a collective decision-making procedure in which each participant is given an equal budget of voting credits, which they can use to vote for or against a proposal (such as a candidate or issue). Participants can cast as many votes for a given proposal as their budget allows, but the costs increase quadratically, i.e. it costs credits to cast votes for the proposal.
Further reading
Buterin, Vitalik (2019) Quadratic payments: a primer, Vitalik Buterin’s Website, December 7.
Lalley, Steven P. & E. Glen Weyl (2018) Quadratic voting: how mechanism design can radicalize democracy, AEA Papers and Proceedings, vol. 108, pp. 33–37.