What are the two double points (::) for?

Question

I see in some classes something like this:

Illustrative class

class Carro
{
    private static $quantidade = 5;

    public function getQuantidade()
    {
        return self::$quantidade;
    }

    public function comprar()
    {

        self::$quantidade--;
    }
}

What I want to know is this, the operator self calls the variable using two points self::. After all, what is this type of call for? Why was it used in this way and not using the operator $this => $this->$quantidade ?