how to use dispatchQueue.main to pass value out of closures in Swift

i need some clarification regarding passing values out of closures. I think the issues is to use dispatchQueue.main but I cannot understand where and why

here is the code:

import UIKit

class ViewController: UIViewController {
var allCard = [Card]()    
let card = Card(name: "", cost: 0, attack: 0, durability: 0, cardClass: "", cardSet: "", imageURL: "", goldenImageURL: "", type: "", mechanics: [["":""]], howToGetGolden: "")

override func viewDidLoad() {
    super.viewDidLoad()
    getCards()
    print(allCard.count)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}

func getCards()  {
    let url = URL(string: "https://omgvamp-hearthstone-v1.p.mashape.com/cards/sets/Classic?mashape-key=....")!        
    let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
        if error != nil { // if urlSession have issues manage the error here
            print(error)
        } else { // if we have data, response, error then continue here
            if let urlContent = data { // if data exist then
                do {
                    let jsonResult = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as! [Dictionary<String, AnyObject>]
                    for cards in 0...jsonResult.count - 1 {
                        if let name = jsonResult[cards]["name"] as? String {    //NAME Optionaml Bindings
                            self.card.name = name
                        } else {
                            self.card.name = ""
                        }                                                       // END Name Optionaml Bindings
                        if let cost = jsonResult[cards]["cost"] as? Int {       //COST Optionaml Bindings
                            self.card.cost = cost
                        } else {
                            self.card.cost = 0
                        }                                                        // END COST Optionaml Bindings
                        if let attack = jsonResult[cards]["attack"] as? Int {    //ATTACK Optionaml Bindings
                            self.card.attack = attack
                        } else {
                            self.card.attack = 0
                        }                                                       // END TTACK Optionaml Bindings
                } catch { //catch error while parsing json
                    print("error")
                }
            }
        } // end if/ELSE
    } // end of task (closure)
    task.resume()
}
}

How should I use dispatchQueue.main here? And is it async or synch? I tried to check on internet but I couldn't find an answer I understood /:

Thanks for any help