カウント画面を作る上でつまづいています

カウントダウン画面を作っています。
circleAnimationメソッドの中の
drawAnimation.setValue(layer, forKey: "animationLayer")
の意味がわかりません。
layerとは何をさしているのでしょうか？DidStopメソッドで取り出していますが、取り出すことで具体的に何ができるのでしょうか。
よろしくお願いします。

swift
1import UIKit
2
3class StartCountDownViewController: UIViewController, CAAnimationDelegate {
4  
5  func segueToScoreViewController(){
6    self.performSegue(withIdentifier: "toViewController", sender: nil)
7  }
8  
9  var countNumberLabel:UILabel!
10  var circleView:UIView!
11  var countDownNum = 3
12  let countDownMax = 3
13
14
15    override func viewDidLoad() {
16        super.viewDidLoad()
17      
18      //数値ラベル
19      countNumberLabel = UILabel(frame: CGRect(x:0, y:0, width:self.view.frame.width, height:self.view.frame.height))
20      countNumberLabel.font = UIFont(name: "HelveticaNeue", size: 54)
21      //中央ぞろえ
22      countNumberLabel.textAlignment = NSTextAlignment.center
23      countNumberLabel.baselineAdjustment = UIBaselineAdjustment.alignCenters
24      self.view.addSubview(countNumberLabel)
25      
26      //円
27      circleView = UIView(frame:CGRect(x:(self.view.frame.width/2)-100, y:(self.view.frame.height/2)-100, width:200, height:200))
28      circleView.layer.addSublayer(drawCircle(viewWidth: circleView.frame.width, strokeColor: UIColor(red:0.0,green:0.0,blue:0.0,alpha:0.2)))
29      circleView.layer.addSublayer(drawCircle(viewWidth: circleView.frame.width, strokeColor: UIColor(red:0.0,green:0.0,blue:0.0,alpha:1.0)))
30      self.view.addSubview(circleView)
31
32        // Do any additional setup after loading the view.
33    }
34  
35  override func viewWillAppear(_ animated: Bool) {
36    countDownNum = countDownMax
37    countNumberLabel.text = String(countDownNum)
38    circleAnimation(layer: circleView.layer.sublayers[1])
39  }
40  
41  func drawCircle(viewWidth:CGFloat, strokeColor:UIColor) ->CAShapeLayer{
42    let circle:CAShapeLayer = CAShapeLayer()
43    let lineWidth:CGFloat = 20
44    let viewScale:CGFloat = viewWidth
45    let radius:CGFloat = viewScale - lineWidth
46    
47    circle.path = UIBezierPath(roundedRect:CGRect(x:0,y:0,width:radius,height:radius), cornerRadius:radius/2).cgPath
48    circle.position = CGPoint(x:lineWidth/2, y:lineWidth/2)
49    circle.fillColor = UIColor.clear.cgColor
50    circle.strokeColor = strokeColor.cgColor
51    circle.lineWidth = lineWidth
52    return circle
53  }
54  
55  func circleAnimation(layer:CAShapeLayer){
56    //アニメーションを行うプロパティ＝今回はstrokeend(起点と終点を設定)
57    var drawAnimation = CABasicAnimation(keyPath: "strokeEnd")
58    
59    drawAnimation.setValue(layer, forKey: "animationLayer")
60    drawAnimation.delegate = self
61    
62    drawAnimation.duration = 1.0
63    drawAnimation.repeatCount = 1.0
64    drawAnimation.fromValue = 0.0
65    drawAnimation.toValue = 1.0
66    
67    drawAnimation.timingFunction = CAMediaTimingFunction(name:kCAMediaTimingFunctionEaseOut)
68    
69    layer.add(drawAnimation, forKey: "circleAnimation")
70    
71    
72    
73  }
74   func animationDidStop(anim:CAAnimation!, finished flag:Bool ){
75    let  layer:CAShapeLayer = anim.valueForKey("animationLayer") as CAShapeLayer
76    countDownNum -= 1
77    countNumberLabel.text = String(countDownNum)
78    
79    if countDownNum <= 0 {
80      
81    }else{
82      circleAnimation(layer: layer)
83    }
84  }
85  
86
87    override func didReceiveMemoryWarning() {
88        super.didReceiveMemoryWarning()
89        // Dispose of any resources that can be recreated.
90    }
91    
92
93    /*
94    // MARK: - Navigation
95
96    // In a storyboard-based application, you will often want to do a little preparation before navigation
97    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
98        // Get the new view controller using segue.destinationViewController.
99        // Pass the selected object to the new view controller.
100    }
101    */
102
103}
104