The US House is composed of representatives from the states based on their population. Although the Constitution does not mandate single-member geographical Congressional districts, a 1967 federal law does. That law was passed for several reasons, including a desire to protect minority voters, Congressional incumbents' fears of at-large elections, and power struggles between the legislative and judicial branches.
Since populations shift over time and the number of people in each district must be approximately the same, electoral districts are occasionally redrawn. Most states have multiple Representatives in the House, and local & state bodies also undergo occasional redistricting. Typically, in the United States, districts are drawn by state legislatures, which have an obvious interest in maintaining the majority party's power. Many other countries block the legislatures from direct involvement in redistricting, or incorporate a form of proportional representation to mute the impact of a party which holds a narrow majority. They may also require redistricting to respect existing political boundaries, or have multiple representatives per district.
The Constitutionally-mandated census sets the number of Representatives from each state, and is thus also being targeted in struggles over the country's demographic changes.
There are various constraints on how electoral districts can be drawn. Despite this, it is fairly easy to draw districts which favor one party over the order. (You can do it yourself here. ) Modern big-data techniques make such gerrymandering faster and more precise, for a further erosion of individual voters' influence. There are additional possible constraints that could be put on the ways districts are drawn which could help recognize (and potentially ban) district maps drawn primarily for partisan benefit.
This is part of a series on voting. Some additional articles in the series are:
