At a demonstration, many people meet in the open air. They want to say that they are for or against something. They want to impress other people with it. Anyone who demonstrates is a demonstrator.
In a democracy, you are allowed to demonstrate. But you have to abide by the rules. The demonstration must be registered in advance, so you tell the state that you are going to demonstrate on a certain day. For example, you are not allowed to bring weapons.
If the rules are not respected, the police can decide to stop the demonstration. This happens especially when demonstrators become violent: Some demonstrators like to damage things, or they even hit other people.
A particularly large demonstration in Germany took place in Bonn in 1983. At that time, about half a million people were against new missiles. In 1989, there were about the same number of people in Berlin on Alexanderplatz. They wanted to be allowed to speak their minds freely in the GDR. In Austria, 300,000 people met in Vienna in 1993. They wanted to show that one should not be against foreigners.
Sometimes you say demonstration when someone demonstrates something. He demonstrates how a device works. Another word for it is presentation.