Members of Congress are elected to write and pass laws representing the views of their constituents. A constituent is a person who is represented by a specific elected official. For example, Bill De Blasio is the mayor of New York City, so his constituents are the residents of New York City. Every American as of January 20, 2017 is a constituent of President Trump. If you are a representative in the House, you need to support bills that are popular with your constituents. Otherwise, they will be unlikely to vote for you in the future!
This can be more complicated than it seems. Some political scientists feel that representatives have a duty to represent the whole nation, not just their own district, and the best way to do that is to listen to the opinions of their constituents and then use their own best judgment to make final decisions. Members of Congress also have to consider how laws they propose will be viewed by members of their political party. Members who do not vote with their party may find it difficult to get assigned to the committees that interest them.