E.g., let assume you always study 3 hours a day. But if you thought you can score a better result by studying an additional hour (this means 3 hours become 4 hours) 4 weeks before an examination, then your marginal action is an additional hour. Your marginal benefit will be extra points you will receive by studying an additional hour against your usual 3 hours. Hence, this means you as a rational student thinks at the margin.