"All Germans shall have the right to assemble peacefully and unarmed without prior notification or permission. -- In the case of outdoor assemblies, this right may be restricted by or pursuant to a law."
Article 8 guards the rights of all German citizens to assembly. An assembly is a gathering of a group of people who have a common political goal. That means, for instance, a group of people dancing together or sitting in a cinema is not an assembly.
People can gather indoors or outdoors. Gatherings outside, i.e. "demonstrations" or "protest" need to be registered. They may be banned occasionally, for instance, if the city is afraid of unrest or violence. But that is rare- In principle, the state must ensure that demonstrations can take place. So often another location or another day is sought and picked.
Carrying weapons is prohibited at assemblies. But people can speak freely, and, e.g. criticise the state.
Article 8 is a "civil right"; so it only applies to German citizens. However, non-Germans can also assemble in Germany, as the European Convention on Human Rights and the UN Convention on Human Rights grant them the right.