Profit is the residual, or what's left over, between total revenue and total cost.
Another way of thinking about marginal revenue is that it is the additional amount of revenue that a firm receives by selling one more unit of output. Again, another way of thinking about marginal cost is that it is the additional cost that a firm pays by selling one more unit of output. In this case the firm is adding to its profits. In terms of a firm, the firm is better off since the firm can increase its profits by increasing output. In this case the firm is incurring more additional costs than additional revenue.