Generally speaking, marginal benefit is the difference (or change) in what you receive from a different choice. From a consumer's point of view, marginal benefit is the additional satisfaction of one more item purchased. From a business' point of view, marginal benefit is the additional revenues received from selling one more item.
Suppose you're considering membership at the local recreation center. The basic membership gives access to the swimming pool, while the full membership gives access to the swimming pool and the weight room. What is the difference between the two memberships? Since both give access to the pool, the marginal benefit of full membership is access to the weight room.
The amount of benefit a person receives from a particular good or service is subjective; one person may get more satisfaction or happiness from a particular good or service than another. For example, you might enjoy ice cream more than your friend who is allergic to dairy. The amount of benefit you get can also change. For example, you might enjoy the ice cream more on a hot day than on a cold day. This doesn't make it any less real, however.