Phylaki is a word from Ancient Greece that means "prison". It was a place where people who did something wrong or against the law were put to stay as punishment.
Think of it like your room, but instead of time-out, it's a place where grown-ups might put you if you did something naughty.
Phylaki was a bit like that, but instead of just staying there until you learn your lesson, you might stay there for a very long time, or even forever! It was a serious punishment for people who did really bad things.
People who were in phylaki probably didn't have much fun because they couldn't play with their friends or do what they wanted to do. They had to stay in a small room all by themselves, or with other people who were also being punished.
So, remember to always be good so you don't end up in phylaki!