added comments to the undo method

src/backend/Board.java

@@ -261,41 +261,41 @@ return false; }
 	 * Reverts the board state back one move.
 	 */
 	public void undoLastMove() {
-	    if (previousStates.isEmpty()) {
+	    if (previousStates.isEmpty()) {  //If previousStates is empty, so we just print a mess
 	        System.out.println("There are no moves to undo.");
 	        return;
 	    }
 
-	    // 1) Pop off the most recent snapshot
-	    Board prev = previousStates.remove(previousStates.size() - 1);
-
-	    // 2) Restore the board array
+	    // Pop off the most recent snapshot
+	    Board prev = previousStates.remove(previousStates.size() - 1); //previousStates is stack of full-board copies taken before each move.
+                                                                    //By removing the last entry,getting back a Board object (prev) that represents 
+	    // Restore the board array                                 the entire game state just before the move we want to undo.
 	    this.board = new Piece[width][height];
-	    for (int x = 0; x < width; x++) {
-	        for (int y = 0; y < height; y++) {
+	    for (int x = 0; x < width; x++) {            //We re-create this.board (your 2D array of Piece references) and copy each cell from prev.board.
+	        for (int y = 0; y < height; y++) {      //That means any piece that was on e4 in the snapshot winds up back on e4 now
 	            this.board[x][y] = prev.board[x][y];
 	        }
 	    }
 
-	    // 3) Restore all other game‐state fields
+	    // Restore all other game‐state fields, Copy every field from that snapshot back into THIS:
 	    this.chosenPiece = prev.chosenPiece;
 	    this.turnNumber= prev.turnNumber;
 	    this.isTurnWhite= prev.isTurnWhite;
 	    this.lastMove= prev.lastMove;
 	    this.highlightedSquares = new ArrayList<>(prev.highlightedSquares);
 
-	    // 4) **Critical**: reset each Piece’s own coordinates to match its array slot
+	    // reset each Piece’s own coordinates to match its array slot
 	    for (int x = 0; x < width; x++) {
 	        for (int y = 0; y < height; y++) {
-	            Piece p = this.board[x][y];
+	            Piece p = this.board[x][y];  // we grab whenever is at the xy coordinates and store it in p varaible
 	            if (p != null) {
-	                p.setX(x);
-	                p.setY(y);
+	                p.setX(x); //By writing those indices back into p via setX(x)/setY(y)
+	                p.setY(y); // we restore each piece’s private coordinates to their pre-move values.
 	            }
 	        }
 	    }
 
-	    // 5) Clear any lingering selection or highlights
+	    // Clear any lingering selection or highlights
 	    this.chosenPiece = null;
 	    this.highlightedSquares.clear();
 	}