Sarita is better off studying for another hour.
Marginal benefit is the additional satisfaction one gains from an additional unit of an activity.
Marginal cost is the additional cost from an additional unit of an activity.
The marginal benefit of studying is grater than the marginal cost means that adding another hour of studying will increase its benefits. In other words, the gained knowledge during the one hour of study is greater than the effort Sarita had to put into studying. If the effort was greater than the benefit, the marginal cost would be grater too and there wouldn't be any reason to study for additional time.