Okay, so let's start with some big words: fascist and liberation. Fascism is a really bad political idea that says that only certain people are allowed to have power and everyone else is supposed to follow them no matter what they say. It's like having a mean bully in charge of everything, and it's not fair or kind. Liberation means freeing people from something that's holding them back or hurting them, like if a superhero saves people from a bad guy.
Now, the anti-fascist assembly is a group of people who don't like fascism and want to stop it from taking over their country, which is called Serbia. They're kind of like superheroes, but instead of using muscles or superpowers, they use their voices and actions to stand up against fascism and help people who are being hurt by it. They might have meetings where they talk about what they can do to stop fascism, and they might make signs or go to protests to show that they don't want fascism to be in charge.
The "national liberation" part means that they want to help Serbia become a better, freer place where everyone can have a say and be treated equally, without any mean bullies in charge. So, they're working towards a better future for everyone in their country, and that's a really important and brave thing to do.