What is a congressional district?

A U.S. congressional district is one of the 435 geographical areas into which the United States is divided which is represented by one of the 435 voting members of the U.S. House of Representatives.

People who live inside the borders of the congresional district elect one of the representative in your state. That particular congressman (representative) will try to spend his efforts and money in your district to convince you to vote for him/her.