explain

[Cutefriendzoned]

Names beginning with two underscore characters or with an underscore character
followed by an uppercase letter are reserved for use by the implementation—that is,
the compiler and the resources it uses.Names beginning with a single underscore
character are reserved for use as global identifiers by the implementation

can someone please explain this ?

does __x means it's a global variable ? and what about _ ?

[Mats]

can someone please explain this ?

To avoid naming conflicts.

does __x means it's a global variable ?

No.