Okay kiddo, so you know how some people can be really mean to other people just because they look, talk, or act differently? That's called prejudice, and it's a bad thing. There were some people in history who were really prejudiced against certain groups of people, like the Nazis were against Jewish people.
Now, there was also a time when people in the Soviet Union and some other countries believed that there was something called "social fascism." They thought that there were some socialist or communist parties that were actually really bad, just like the Nazis were. They believed that these parties pretended to be for the working class, but they were actually secretly working with the rich people to keep the working class down.
But here's the thing, kid: most scholars and historians today don't think that social fascism was a real thing. They think it was just a misunderstanding of what fascism really was. Fascism is a political system that values extreme loyalty to a leader, strict control of the media, and militarization of the society. Most socialist and communist parties don't believe in any of those things.
So when people talked about "social fascism" before, they were mostly just confused. It's important to always be careful with labels like that, and to make sure we really understand what we're saying!