What does senators are elected at large mean?

When an elected official is "elected at large" that means that they are elected by the entire body of voters rather than by individual districts. In other Words, every eligible voter in the entire state can vote on each Senator. Representatives in the House, on the other hand, are not elected at large. Individual districts within each state vote for which Congressman will represent them in the House. A voter living in district 1 cannot vote for a Representative running in district 2. There are exceptions where a state can have a congressman at-large, for example, Delaware, where the entire state votes for the representative. Because Senators are "elected at large," however, districts don't matter.