Very simple. If marginal revenue (benefit) is below marginal cost, that means you made a loss on your last unit of output. You can therefore increase your profit by reducing output.
This is what I have written in response to a similar question.
Marginal cost and benefit, in a common sense, are the cost and benefit of the last unit produced. If this benefit is less than the cost, one can decide not to produce this unit. Since by doing so (by not producing this unit) net loss can be reduced.
