A lobbyist is someone hired by a business or a cause to persuade legislators to support that business or cause.
Lobbyists get paid to win favor from politicians. For example, oil companies send lobbyists to Washington to try to make life easier for oil companies. Sometimes they do it by making a great case for their cause, but often it involves fancy dinners and golf outings. If that sounds kind of shady, it is. But remember that women's rights groups and cancer research foundations have lobbyists, too — it's just one way to get your voice heard on the Hill.