During a demonstration, a lot of people gather open air. With this, they want to say that they are for or against something. With a demonstration, they want to influence others. Whoever demonstrates is a demonstrator.
In a democracy, you can organize a demonstration but you have to tell the government that this demonstration is going to happen. Another thing is that the demonstrators are not allowed to bring weapons.
If rules are not abided by, then the police can decide that the demonstration stops. This happens mainly if a demonstration turns violent or demonstrators start destroying things.
A well-known demonstration happened in Germany in 1983, where around 500.000 people demonstrated against new missiles. In 1989, about half a million people from the GDR demonstrated for freedom of speech in Berlin. In Austria, in 1993 about 300.000 people demonstrated to say that immigrants should be treated equally.
Demonstration is also sometimes described as a presentation. To demonstrate how a machine functions. Another word for that is presentation.