Adding Integers and Strings (converting back and forth)

Ok, so i am on a Swift course and wanted to practice some of my "projects" to see if i got everything down. I ran into an error that i don't quite understand. I also realize that there is probably a much better way to do this, but here it is :)

I have 2 labels which show a random number at the press of a button and a text field that shows a result of adding those two numbers.

Main issue is that i get a String from the random number generator so i can put it inside the label, and then i need to convert it to Int so i can ad it, and then back to String so i can put that in the text field.

I managed to make it work, after xCode suggested i add "!" in two places as you can see below. But i have no idea what it means ? Error before that was: "value of optional type string not unwrapped, use "!" or "?" "

@IBAction func btnPress(_ sender: Any) {
        numberLabel.text = randomNum()
        numberLabel2.text = randomNum()
        result = Int(numberLabel.text!)! + Int(numberLabel2.text!)!
        txtField.text = String(result)
    }

My random function just in case you need it:

func randomNum() -> String{
// OPTION 1
//let number = GKRandomSource.sharedRandom().nextInt(upperBound: 1000)
    let number = arc4random_uniform(6)
    return String(number)
}

I tried to search online and get some sense of it, but still not getting it 100%. I hope someone can let me know what it means and when it is used.

Cheers !