The term "Marginal" in economics is used extremely often. What it means, is essentially the next additional unit, product, person, or whatever else you're associating the term with.
For example, say you're reading an economics textbook and you come across the term "Marginal Profit." What this means, is the profit you will gain from selling one additional unit of good, after taking into account the marginal cost as well.
Another example, "Marginal Utility" can be explained as the additional utility a consumer receives from consuming one more additional unit of good.
The most used terms would most likely be Marginal cost, and marginal revenue. Businesses will place a lot of importance on these type of terms because companies not only want to maximize and minimize profit, but they also want to be efficient with what they do. If a marginal cost is increasing, while marginal revenue is decreasing, it wouldn't make any sense to continue to produce past the point where marginal cost is more than marginal revenue. Thus, by looking at the margins and not just at the final big picture of revenue vs cost, companies are able to adjust more quickly in the short run, and help their business reach optimal efficiency.