Many people think that “one person, one vote” means exactly that: one person gets one vote, no more, and no less. In other words nobody gets to vote twice, and everybody should get to vote once. However, what ‘one person, one vote’ actually means is that everybody’s vote should be equal. That any person’s vote should count just as much as any other person’s vote. More specifically, that no person’s vote should have more or less weight than anyone else’s vote. But that’s often not how it is, especially when it comes to redistricting.
As the Constitution Project explains, the term ‘one person, one vote’ was first articulated (at least with any teeth) in 1962, by Chief Justice Earl Warren. As the Constitution Project explains “Starting with the Court’s 1962 decision in Baker v. Carr and culminating in 1964 with the case of Reynolds v. Sims, the value of ‘One person, one vote,’ once brought to light, seemed so profoundly rooted in the Constitution its practice became ‘inevitable.'”
The purpose of the ‘one person, one vote’ concept was to establish equality in voting and representation. In other words, everybody’s vote should have equal weight in the outcome of the vote.
However redistricting can make this nothing more than a pipe dream. Where this plays out most clearly is in elections for state and local representatives.
In order to not pick on any political party, let’s say there are two political parties: the Shirts and the Skins. And let’s say there is a state-wide election in the state of Globista.
Currently the Shirts have control of the Globista state legislature. They want to be sure that they maintain control after the upcoming election and they do it by redistricting. There are two primary ways that they can accomplish this, known as “cracking” and “packing”.
To crack through redistricting the Shirts will draw the new district lines so that they break up the larger pockets of Skins into different districts where the new smaller pockets of Skins will be outnumbered by the numbers of Shirts in the newly redrawn districts. This means that in the upcoming election each newly drawn district will be heavily weighted to Shirts, with the Skins’ votes in each newly drawn district heavily diluted – in other words having less weight.
On the other hand, to pack through redistricting means that the Shirts will redraw the district lines so that more of the Skins are contained within one district, allowing the Shirts to sweep the other districts (think of the phrase “We didn’t cross the border, the border crossed us.”) Again, the votes of the small minority of Skins in the other districts are now far outweighed by the majority weight of the votes of the Shirts.
In both situations the Shirts have now devalued each individual Skin’s vote, making each individual Skin’s vote, arguably, not equal to either each individual Shirt’s vote or, in the case of district packing, other Skins’ votes (where the votes of those Skins outside the packed district will have little weight against the large number of Shirts’ votes). In a blatant real world example, in 2016, in North Carolina, which then had a majority Republican state legislature, the redistricting committee chair declared “I propose that we draw the maps to give a partisan advantage to 10 Republicans and 3 Democrats, [but only] because I do not believe it’s possible to draw a map with 11 Republicans and 2 Democrats.”
For an excellent, plain English, and more in-depth explanation of redistricting, with real world examples, see this article from Loyola Law School.