Tsumi is a Japanese word that means doing something wrong or bad. It's like when you get in trouble for breaking a rule or doing something naughty. In Japan, it's a very big deal to not follow the rules or behave properly, and they have a lot of different words to describe different kinds of bad behavior.
For example, if you take something that's not yours, that's called "stealing," which is a type of tsumi. If you hurt someone's feelings on purpose, that's called "bullying," which is also a type of tsumi. Even if you accidentally do something wrong, like spilling juice on the carpet, that's still a type of tsumi because you weren't careful enough.
It's important to try to be a good person and not do tsumi, so you don't get in trouble and make people upset.