Blue flu refers to a situation where a large number of police officers in a particular area call in sick at the same time, leaving the police force short-staffed. It is a kind of protest by the police officers to show their disagreement with something happening at their workplace or the government.
When police officers feel that they are not being treated fairly, or their wages or working conditions are being neglected, they may all decide to take a sick day at the same time. this action is called blue flu because it is a way of the police officers showing solidarity with each other, and blue is the traditional color of a police uniform.
However, calling in sick when you're not really sick is not a good thing to do. It's not honest, and it can cause harm to other people. So, while it might be tempting to take part in a blue flu, it's important to remember that there are other, more ethical ways to protest and make your voice heard.