Can someone tell me what emotional regulation means ?

Its a core topic of ADHD . But I don't understand what it means. ADHD people have trouble self regulating emotions. It says.
But I have seen neurodivergent people behaving much well and not harming others. I am one of them.
My boyfriend is a NT and he just don't know how to express his anger in a healthy way. Just like so many of my NT friends.
I am not judging them but I am trying to understand the meaning of Regulating emotions.