Homo sacer is a big and difficult word that means "sacred man" or "cursed man." In ancient Rome, it was a rule that if someone did something very wrong, they could be made into a "sacred man" who was not allowed to be killed, but also not protected by law.
This means that they were kind of stuck in the middle, unable to live a normal life, but also not allowed to be killed like a normal person, even if they did something very bad. They were basically outcasts, and nobody wanted them around.
This idea of homo sacer has been studied by philosophers and scholars to try to understand why and how societies go about defining who is "sacred" or "cursed" and what it means for those people. It is often used to talk about people who are discriminated against or treated unfairly by society because of their race, gender, or other personal factors.
So, in short, homo sacer is a way of talking about people who are cast out of society and how they are treated differently than others.