setcolor(BLACK);
    rectangle(snake.crd[snake.node].x,snake.crd[snake.node].y,snake.crd[snake.node].x+SNAKELEN,snake.crd[snake.node].y+SNAKELEN);
    setcolor(WHITE);
    rectangle(snake.crd[snake.node-1].x,snake.crd[snake.node-1].y,snake.crd[snake.node-1].x+SNAKELEN,snake.crd[snake.node-1].y+SNAKELEN);

这两句话是什么意思