Okay, imagine you're playing a game where there are two teams: the good guys and the bad guys. In this game, the good guys want to be very good and not do anything bad, while the bad guys don't care if they do bad things.
Now, there was a group of people called the Cathars who believed that they wanted to be like the good guys in this game. They wanted to be as good as possible and not do anything bad.
But the Cathars believed that it was really hard to be good all the time. Sometimes, they might accidentally do something bad, even if they didn't mean to. So they came up with an idea called "perfect."
To be "perfect," the Cathars believed that you had to be really, really good all the time. You had to be perfect in everything you did, and never do anything even a little bit bad.
This was a really hard thing to do, but the Cathars believed it was worth it. They thought that by being perfect, they could get closer to God and to heaven.
So that's what "Cathar perfect" means. It's the idea that you can be so good that you never do anything bad, and that by doing that, you can get closer to God.