What is purpose of "::" before a "-webkit" css declaration

Problem

Recently I read a CSS declaration that started with ::. Can you tell me its purpose?

Example:

::-webkit-scrollbar {
  width:0;
}

It is used here for example.

Problem courtesy of: meo

Solution

That's a CSS pseudo-element.
The :: is part of the selector, just like : is part of the :focus selector.

Solution courtesy of: SLaks

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