Explanation - MVC

Hi.

I am following the Microsoft tutorial to create a music store. Link : http://www.asp.net/mvc/tutorials/mvc-music-store.  

I'm at the point where they are creating the shopping cart, and on the shopping cart class they have the following code:

public void AddToCart(Album album)
        {
            //Get the matching cart and album instances
            var cartItem = storeDB.Carts.SingleOrDefault(
                c => c.CartId == ShoppingCartId
                    && c.AlbumId == album.AlbumId);

            if (cartItem == null)
            {
                //Create a new cart item if no cart item exists
                cartItem = new Cart
                {
                    AlbumId = album.AlbumId,
                    CartId = ShoppingCartId,
                    Count = 1,
                    DateCreated = DateTime.Now
                };
                storeDB.Carts.Add(cartItem);
            }
            else
            {
                //if the item does not exist in the cart then add one to the quantity
                cartItem.Count++;
            }
            //Save changes
            storeDB.SaveChanges();
        }

Can anyone explain this to me?  Especially the "SingleOrDefault" part.