In the real world, the client would be the customer, the owner of the thing being built, and the primary "stake-holder". They fund the projects, and they have typically veto power what the project, but not how it is done on a day to day. This person's opinions and feelings matter, because the team's responsibility is to build something that the client will be happy with. This person could be an "end user" of the project, but also could be just an interested party.