Understanding what && does [duplicate]

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• What does T&& (double ampersand) mean in C++11? 3 answers

If I have

int x = 25;
int& y = x;

I understand that x and y point to the same memory location. However, when I have:

int x = 25;
int&& y = x;

I get:

error: expected unqualified-id before '&&' token

What is the explanation in terms of lvalue and rvalue? I cannot wrap my head around the concept and how it is applied here.

Also, is it true that if you want to reference a number, you should only be able to do it using something like const int& z = 25 and never int& z = 25 because of the reason that you are referencing a number and numbers don't change?