ios – return self.desk[number]![alpha]. Please assist me perceive the which means of this code

Good night pricey colleagues, please assist me determine it out. Easy methods to perceive this a part of the code. Why”!”it’s situated between two brackets []![].

What’s the which means of this expression?

self.desk[number]![alpha]

class GameDesk {
    var desk : [Int: [String: Chessman]] = [:]

    init() {
        for i in 1...8 {
            desk[i] = [:]
        }
    }

    subscript(alpha: String, quantity: Int) -> Chessman? {
        get {
            return self.desk[number]![alpha]
        }
        set {
            if let chessman = newValue {
                self.setChessman(chess: chessman, coordinates: (alpha, quantity))
            } else {
                self.desk[number]![alpha] = nil
            }
        }
    }

    func setChessman(chess: Chessman, coordinates: (String, Int)){
        let rowRange = 1...8
        let colRange = "A"..."Z"
        if(rowRange.comprises(coordinates.1) && colRange.comprises(coordinates.0)) {
            self.desk[coordinates.1]![coordinates.0] = chess
            chess.setCoordinates(char: coordinates.0, num: coordinates.1)
        } else {
            print("Coordinates out of vary")
        }
    }
}

The which means of this piece of code doesn’t attain me