Java Synchronization for Thread Safety

In the GameBoard class shown below, what is the primary purpose of the synchronized block within the play method?

public class GameBoard {
    private int[][] board = new int[3][3];

    public void play(int row, int col, int player) {
        synchronized(board) {
            if (board[row][col] == 0) {
                board[row][col] = player;
            }
        }
    }

    public void printBoard() {
        for (int[] row : board) {
            for (int cell : row) {
                System.out.print(cell + " ");
            }
            System.out.println();
        }
    }

    public static void main(String[] args) {
        GameBoard game = new GameBoard();
        Thread t1 = new Thread(() -> game.play(1, 1, 1));
        Thread t2 = new Thread(() -> game.play(1, 1, 2));
        t1.start();
        t2.start();
    }
}