MC is the additional cost of producing one more unit of output
MR is the additional revenue obtained from selling one more unit of output
MR > MC, the firm can increase its profits by producing more output
MR < MC, the firm can increase its profits by producing less
The firm will continue to expand output as long as marginal revenue is greater than marginal cost because doing so adds to the firm's profits. Once MR=MC, any further increase in output lowers the firm's profits.