To elaborate slightly, there's three components here:

    let x = 5;
    let mut y = 6;

The let statement binds a variable to a value:

  * x and y are the variables
  * 5 and 6 are values
  * let does the binding

You can have an immutable binding, like x, or a mutable binding, like y. But most people turn "a bound variable" into "a variable" (or "an immutable variable") and "a mutably bound variable" into a "mutable variable".