That’s the rule requiring that to the extent possible voting districts be drawn with same-sized populations — so that one person’s voting power is roughly equivalent to another person’s within a state.
Here’s more from Rick Hasen at the Election Law Blog:
In a surprise move, the Supreme Court agreed to hear an appeal from a three judge court in Evenwel v. Abbott, a one-person, one vote case involving the counting of non-citizens in the creation of electoral districts. Ed Blum, the force behind the Fisher anti-affirmative action case and the Shelby County case striking down a key portion of the Voting Rights Act is also behind this case. The question involves whether Texas can draw districts using total population rather than total voters, an issue especially important given non-citizen Latinos living in parts of Texas. The claim is that representatives from these areas with non-citizens get too much moving power. A ruling in favor of the challengers would be a boost for areas with fewer numbers of non-citizens living there.