Lobbyist are people who try to influence politician in a variety of ways. They can be professionals who are part of an organisation and their expertise is influencing politicians.

The term might originate from the lobby of the House of Commons where people might meet their MP- If you go to see you MP you might say you are 'lobbying' them. Or it might originate in the USA form the lobby of the Willard Hotel- where people met politicians in Washington to seek favours.

How can politicians be influenced? 

Money, bribes, help, advice, persuasive arguments?