Okay kiddo, so femonationalism is when people try to use feminist ideas to justify being mean or discriminatory towards people who are different from them. It's kind of like when a kid says "I don't want to invite so-and-so to my birthday party because they're different and I don't like them."
Unfortunately, some adults do this too. They might say things like "I support women's rights but only for some women, not for immigrant women or women who wear different clothes than me." This is not fair or kind.
Real feminism is about making sure everyone has equal rights and equal opportunities, no matter who they are or where they come from. We should always try to include and respect people who are different from us, not push them away.