To shuln means to do something that you should not do, or to not do something that you should do.
Shuln is usually a bad thing. It can make people feel guilty or regretful.
Examples:
I shuln have eaten all the cookies.
You shuln't forget to do your homework.
She shuln't have said that mean thing.
He shuln't have taken the last piece of cake!
I shuln't have stayed up all night watching TV.
If you shuln't have done it, you should say sorry.