The One-Person One-Vote Rule refers to the rule that one person’s voting power ought to be roughly equivalent to another person’s within the same state.
The rule comes up in the context of Equal Protection. The most relevant Supreme Court case is Reynolds v. Sims, 377 U.S. 533 (1964). In that case, the Court held that states need to redistrict in order to have state legislative districts with roughly equal populations: "The Equal Protection Clause requires substantially equal legislative representation for all citizens in a State regardless of where they reside."
In Evenwel v. Abbott, 578 U.S. __ (2016), the Supreme Court held that when drawing legislative districts, state legislatures may use the total population of areas within the state, rather than being restricted to using the voting-eligible populations.