Roughly speaking, lobbying is when any non-politician tries to influence the opinion of politicians.
Let's say you would like your favorite forest to be declared a national park. What would you do? You could write a letter to the responsible politicians and ask them to do this. This is a form of lobbying. However, as a Joe Average, it is unlikely that they will listen to you.
But when a well-known environmental group, like Greenpeace or the World Wildlife Foundation, would ask for it, they might pay more attention. Such non-governmental organizations (NGOs) usually have established contacts with politicians to tell them their opinion about current events, and the politicians usually take concerns from such organizations seriously. First, because they know such organizations have experts on their pet issue who usually are aware of the full ramifications of their proposal, and second because NGOs represent not just a single voter, but the interests of all their members and often a much larger number of non-member sympathizers they would be able to mobilize against politicians who act against their interests. This allows such groups to engage in lobbying much more efficiently than private people could.
But a good(!) politician wouldn't base their decisions on the opinion of just a single organization. Before proposing a law, yet alone cast their vote, they would also want to listen to contrary viewpoints. In this example they will certainly want to listen to what the representative of the local industry think about this. Some might provide further arguments for it (like the tourism industry), others will be vehemently against it (like any polluting industry which would then need to shut down).
The idea of lobbying itself isn't inherently undemocratic. A politician should not just follow their gut instinct but should consider all the conflicting interests before making their decision. Meeting with people who represent these interests is a useful way to ensure that this happens.
However, the reason why lobbying has such a bad reputation, is because those lobbyists who represent businesses are usually far better funded than those who represent NGOs. While NGOs are often underfunded and rely on volunteers who work in their free-time, industry lobbyists can afford whole teams of full-time employees who do nothing all day except figuring out the best way to convince politicians to do exactly what their employers want. This often leads to a perceived over-representation of economic interests in the decision-making process of politicians.
Also, when lobbyists have access to lots of funding, lobbying can sometimes become dangerously close to bribery. For example when companies promise hefty donations to the parties of politicians in exchange for favorable decisions.
That's why there are frequently demands for legislation to make lobbying more transparent. By making it clearer which politician gets influenced by whom, the whole political process which leads to a decision becomes more visible to the public, and politicians are motivated to listen to contrary viewpoints to avoid accusations of being influenced by just a single organization.