In this case, x is an optional, eg. Int? (if you are unfamiliar with Swift, this means either an Integer or the special value nil)

"guard let x = x where x > 0 else {}" first unwraps the optional (check if it is not nil), then it checks if the value is greater than 0.

This is the nice thing about "guard let ...". Below the guard statement, x is no longer an optional.