Basic Stuff on Arrays

I am learning from a book on unity and it used this

 var aGrid:Card[,];//2d array to keep track of the shuffled, dealt cards

I understand that :Cards means that it will only hold instances of the Cards class, but I don't understand what the comma in there means. There are no "," in my Card class. Can anyone explain what this does to me?

Here is the full code if you need it:

 #pragma strict
 
 class Card extends System.Object {
     var isFace:boolean = false;
     var isMatched:boolean = false;
     var img:String;
     
     function Card() {
         img = "robot";
     }
 }
 import System.Collections.Generic;
 var cols:int = 4; //number of collumbs
 var rows:int = 4; //the number of rows in a card grid
 var totalCards;int = 16;//4*4=16, there are sixteen slots in our grid for 16 cards
 var matchesNeededToWin:int =  totalCards * 0.5;//two cards is one match
 var matchesMade:int = 0;//At the outset, the player has not made any matches
 var cardW:int = 100; // Each card is 100px by 100px
 var aCards:List.<Card>;//We'll store all the cards we created in this list.
 var aGrid:Card[,];//2d array to keep track of the shuffled, dealt cards
 var aCardsFlipped:List.<Card>;//stores flipped cards
 var playerCanClick:boolean;//prevent players from clicking wrong buttons
 var playerHasWon:boolean = FalseString;//store wether or not the player has won