Yom Kippur Katan is like a little, mini version of a big day called Yom Kippur that happens once a year. It's like practicing for the big day.
On Yom Kippur, people who follow the Jewish religion fast (which means they don't eat any food) and ask for forgiveness for all the things they did wrong in the past year. They also ask to be written in a special book called the Book of Life, so they can have a happy, healthy, and peaceful year ahead.
Yom Kippur Katan is like a small, mini Yom Kippur that happens every month, just to remind people to think about the things they did wrong and ask for forgiveness. People don't fast like they do on the big day, but they do special prayers and try to make things better with anyone they might have hurt or upset.
It's like a little reminder to be a better person every month, instead of waiting a whole year to try to do better.