A box is like a single-element vector. It can print as a quoted #& followed by the printed form of the boxed value. A #& form can also be used as an expression, but since the resulting box is constant, it has practically no use.
> (define b (box "apple")) > b
> (unbox b)
> (set-box! b '(banana boat)) > b
Boxes in The Racket Reference provides more on boxes and box procedures.