'NoteDetail' isn't defined for the class 'NoteListState'.

I am getting this error:
'NoteDetail' isn't defined for the class 'NoteListState'.

• 'NoteListState' is from 'package:notes/screens/note_list.dart' ('lib/screens/note_list.dart').
  Try correcting the name to the name of an existing method, or defining a method named 'NoteDetail'.

The function where this error originates is this one:
void navigateToDetail(String title) {
Navigator.push(context, MaterialPageRoute(builder: (context) {
return NoteDetail(title);
}));
}
}

[shabekpsr]

I can't run the code. due to context error. I cant understand this context. please explain to me what is context. why it disturbed me always.