My code works, I just don't understand WHY it works

 #pragma strict
 
 import System.Collections.Generic;
 
 var cols:int = 4; // the number of columns in the card grid
 var rows:int = 4; // the number of rows in the card grid
 var totalCards:int = 16;
 var matchesNeededToWin:int = totalCards * 0.5; // If there are 16 cards, the player needs to find 8 matches to clear the board
 var matchesMade:int = 0; // At the outset, the player has not made any matches
 var cardW:int = 100; // Each card's width and height is 100 pixels
 var cardH:int = 100;
 var aCards:List.<Card>; // We'll store all the cards we create in this List
 var aGrid:Card[,]; // This 2d array will keep track of the shuffled, dealt cards
 var aCardsFlipped:List.<Card>; // This generic array list will store the two cards that the player flips over
 var playerCanClick:boolean; // We'll use this flag to prevent the player from clicking buttons when we don't want him to
 var playerHasWon:boolean = false; // Store whether or not the player has won. This should probably start out false :)
 
 function Start ()
 {
     playerCanClick = true; // We should let the player play, don't you think?
 
     // Initialize some empty Collections:
     aCards = new List.<Card>(); // this Generic List is our deck of cards. It can only ever hold instances of the Card class.
     aGrid = new Card[rows,cols]; // The rows and cols variables help us define the dimensions of this 2D array
     aCardsFlipped = new List.<Card>(); // This List will store the two cards the player flips over. 
 
     BuildDeck();
     
     // Loop through the total number of rows in our aGrid List:  
     for(var i:int = 0; i<rows; i++)
     {
 
         // For each individual grid row, loop through the total number of columns in the grid:
         for(var j:int = 0; j<cols; j++)
         { 
             var someNum:int = Random.Range(0,aCards.Count);
             aGrid[i,j] = aCards[someNum];
             aCards.RemoveAt(someNum);
         }
     }
     //for(i=0; i<1000; i++)
     //{
     //    Debug.Log(Random.Range(0,10));  // Uncomment to play around with Random
     //}
 }
 
 function OnGUI ()
 {
     GUILayout.BeginArea (Rect (0,0,Screen.width,Screen.height));    
     BuildGrid();
     GUILayout.EndArea();
     //print("building grid!");
 }
 
 function BuildGrid()
 {
     GUILayout.BeginVertical();
     GUILayout.FlexibleSpace();
     for(var i:int=0; i<rows; i++)
       {
         GUILayout.BeginHorizontal();
         GUILayout.FlexibleSpace();
         for(var j:int=0; j<cols; j++)
         {
               var card:Card = aGrid[i,j];
               var img:String;
               if(card.isFaceUp)
               {
                   img = card.img;
               }    else {
                   img = "wrench";
               }         
               if(GUILayout.Button(Resources.Load(img), GUILayout.Width(cardW)))
               {
                    if(playerCanClick)
                   {
                   FlipCardFaceUp(card);
                 //Debug.Log(card.img);
                   }
             }
         }
         GUILayout.FlexibleSpace();
         GUILayout.EndHorizontal();   
       }
       GUILayout.FlexibleSpace();
       GUILayout.EndVertical();
 }
 
 class Card extends System.Object
 {
     var isFaceUp:boolean = false;
     var isMatched:boolean = false;
     var img:String;
     
     function Card(img:String)
     {
         this.img = img;
     }
 }
 
 function BuildDeck()
 {
     var totalRobots:int = 4; // We've got four robots to work with
     var card:Card; // this stores a reference to a card
     
     for(var i:int=0; i<totalRobots; i++)
     {
         var aRobotParts:List.<String> = new List.<String>();
         aRobotParts.Add("Head");
         aRobotParts.Add("Arm");
         aRobotParts.Add("Leg");
         for(var j:int=0; j<2; j++)
         {
             var someNum:int = Random.Range(0, aRobotParts.Count);
             var theMissingPart:String = aRobotParts[someNum];
             
             aRobotParts.RemoveAt(someNum);
             
             card = new Card("robot" + (i+1) + "Missing" + theMissingPart);
             aCards.Add(card);
                 
             card = new Card("robot" + (i+1) + theMissingPart);
             aCards.Add(card);
         }    
     }
 }
 
 function FlipCardFaceUp(card:Card)
 {
     card.isFaceUp = true;
     if(aCardsFlipped.IndexOf(card) < 0 )
     {
         aCardsFlipped.Add(card);
     }
     Debug.Log(aCardsFlipped.IndexOf(card));
     
     if(aCardsFlipped.Count == 2)
     {
         playerCanClick = false;
         
         yield WaitForSeconds(1);
         
         aCardsFlipped[0].isFaceUp = false;
         aCardsFlipped[1].isFaceUp = false;
         
         aCardsFlipped = new List.<Card>();
         playerCanClick = true;
     }
     
 }

What we're going to look at is the FlipCardFaceUp function near the end of the code, inside is this little snippet:

if(aCardsFlipped.IndexOf(card) < 0 )

{

aCardsFlipped.Add(card);

}

I've been trying to figure out these 4 lines of code for maybe 3 hours and I still don't understand what's going on, even though my book explains it, it's still not clicking with me.

This if statement confuses me so much!!

I'll be specific, my book added this if statement because if you click the same card twice it would flip back over anyway, somehow adding this if statement prevents that from happening, but I don't know why.