[disclaimer: still learning Rust]

I believe it actually means that y can be made to reference something other than x.

  let x = 5
  let mut y = &x
  let i = 13
  y = &i

This is similar to C++'s const-pointers and pointers-to-consts, where either a pointer cannot be made to point at something different or the pointer cannot be used to change what it points at.