Imagine you live in a state with three districts and 30 people, 20 democrats and 10 republicans. Normally, you would expect to have 2 democrats elected and 1 republican because thats representative of your state. Pretty standard statistics and expected values.

However, lets say the governor draws new district lines. He crams 12 of the democrats into one group, and 4 democrats and 5 republicans in the other two. Well now the republicans outnumber the democrats and you get 2 republican representatives and 1 democrat. The representation is now no longer is indicitive of the population.