Yes, you have the right idea. How much more of something to get the same measure of utility as from the previous or existing derived marginal utility. In other words, to all things that can be consumed, there comes a point at which the cost exceeds the actual or perceived benefit. To speak of marginal utility is to add fixed units of additional cost to arrive at a point at which derived benefit stops increasing and starts diminishing. Utility can be something measured subjectively, or something tangible such as capital utility to more produce goods that can in fact be sold without adverse discounting.