A more thorough explanation:

The one-person, one-vote rule means that each person's vote should have roughly the same weight as another person's vote in the same state. This rule is important when creating voting laws and redistricting, which can affect the voting power of different groups.

For example, if a state creates a voting law that gives more power to rural voters than urban voters, this would violate the one-person, one-vote rule. Similarly, if a state redistricts in a way that gives one group more voting power than another group, this would also violate the rule.

The one-person, one-vote rule is based on the idea of equal protection, which means that everyone should be treated equally in their voting power. The Supreme Court case Reynolds v. Sims established that states must have legislative districts with roughly equal populations to ensure equal representation for all citizens. In Evenwel v. Abbott, the Supreme Court clarified that states can use the total population of areas within the state when drawing legislative districts, rather than just the voting-eligible population.