How to pass a string to several view controllers?

To move from one view controller to another you can relate them by a segue way, in this way you can pass information from one view controller to another.

Let's say you have a MyViewController1 and you want to pass information to MyViewController2

class MyViewController1: UIViewController {

var infoPrueba: String!

override func viewDidLoad() {
    super.viewDidLoad()
    //Asignamos algún valor a la variable que deseamos
    infoPrueba = "Esta es una info"
}

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "segue" {
        //Obtenemos la referencia del siguiente view controller
        let controller2 = segue.destination as! MyViewController2
        //Aqui pasas la variable de información al siguiente view controller
        controller2.infoDeViewController1 = infoPrueba
    }
}

}

class MyViewController2: UIViewController {

var infoDeViewController1: String!

override func viewDidLoad() {
     super.viewDidLoad()
     //En este punto la variable debe contener la información del 
     anterior view controller
     print(infoDeViewController1)
}

}

There are several ways to share information in different controllers, which one you use depends on what you need. The most common options are:

class Global {
static let sharedInstance = Global()

var anyVariable: String
}

In any class you access the value:         Global.sharedInstance.anyVariable

Clase A Emitirá la notificación
let nc = NotificationCenter.default
nc.post(name:Notification.Name(rawValue:"Cambiounvalor"),object: nil, userInfo: ["valor":1])

Clase B Recibirá la notificación
let myNotification = Notification.Name(rawValue: "Cambiounvalor")
let nc = NotificationCenter.default
nc.addObserver(forName:myNotification, object:nil, queue:nil, using:metodoqueseejecuta)

func metodoqueseejecuta(notification:Notification) -> Void {
guard let index = notification.userInfo?["valor"] as? Int
}