Map of gerrymandering in the United States
An explanation for non-American redditors: Each Congressional district elects one representative, so that more populated states have more than less populated states. This is intended to counterbalance the equal representation in the Senate. However, on many occasions both major parties wildly - wildly - redraw the borders of the districts to encompass as many voters from their party as possible.
For example, Party A redraws the borders in a state so that the people voting for Party B are clustered in one or two districts, and the people voting for Party A are spread out over all of the other districts.
Explains a lot…