What does "[something] is not a joke" mean?

Native speaker’s answer
Rebecca
When someone uses the informal expression "this is not a joke", they are stating that they are being serious, that what they are saying is the truth and not fake or something funny (literally not a joke). Here, the speaker says "this isn't a joke, Gwen", to tell Gwen to be serious and stop joking around. Ex: This isn't a joke. I'm being serious here. Ex: She wasn't joking. It turned out that her crazy story was completely true.