For those not particularly familiar with the concept, it's when a person is paid by some group or person such as an advocacy group to try to encourage some governmental body or group of politicians, or both, to do or not do something. They usually do this with information about the subject matter, surveys they or their client organized or paid for, and in some cases, personal expertise in the field such as if they personally had been a politician or a civil servant in that sector or if they otherwise have experience. In some more corrupt engagements, it may come with gifts or personal relationships like nepotism. Note that the client doesn't themselves want to try to be part of the government or the bureaucracy, at least in the near term, and just wants those who are to shift their stances or maintain a certain strance or action on something.
Lobbying is regulated in various ways across Europe, and different people have different opinions on it from being the right of people to advocate for and against things they want to being little better than bribe-givers and a deeply poisonous aspect of government.