"Man flu" is a term used to describe when a grown-up man gets a cold and acts like it's a really big deal. When someone has a cold, they usually feel pretty yucky and might have a runny nose, sore throat, and a cough.
But with man flu, some people think that men sometimes make a bigger deal out of having a cold than they need to. People might say that men complain a lot and act like they're really, really sick when they just have a cold.
Basically, some people use the term "man flu" to tease men for acting like they're dying when they just have a cold. But it's important to remember that everyone experiences sickness differently, and it's okay to feel really bad when you're sick no matter who you are.