By district, which is all the voters in a smaller geographic area. House Reps are up for re election every 2 years. The intent of the House is to give more representatives to states with higher populations, so for example New York has 26 representatives while Kansas only has 4.
However, instead of purely representing population, the number of House reps has been capped at 435, which weighs heavily in favor of less populated (Republican) states. If that cap was removed, Democrats would have a permanent lock on the House.