In plain language it is the concept that the more you add to an existing property, the less you get in return. Just one example. If you add a room to a house, that may gain a considerable return by way of additional square footage and utility. But, with each additional item you add, the less return you get until, at some point, if you add more, not only will you get less in return, but in fact may cause a loss in overall value due to overimprovement. To quote from "Fundamentals of Real Estate Appraisal, written by Martha Williams & William Ventolo, Jr.................."Improvements to land and structures will eventually reach a point at which they will have no positive effect on property values. As long as money spent on such improvements produce a proportionate or greater increase in income or value, the law of increasing returns is in effect. At the point when additional improvements bring no corresponding increase in income or value, the law of decreasing returns is operating." You should also look at the principle of contribution in assessing this, as well as the principle of conformity, progression and regression.
Don Clark