Can someone explain like I’m 5 what nonverbal is?

I’m ADHD girlie, so fellow ND, but I don’t have autism. As much as I have read about nonverbal online, I don’t understand it.

Is it the inability to speak, the non willingness to speak? Does it only happen in children? Can someone give an example of how this would play out in day to day life? Can it affect non-autistic people too?

I just don’t fully get it.