What Is Utility?
So far, the assumption is that consumption of a good confers positive utility—it makes the consumer happier. This is why economists refer to "goods." There are also items in the world that make the consumer feel worse and promote negative reactions. Economists refer to these sorts of items as "bads." Examples of "bads" are a television set that arrives broken and cigarettes (over time). It is often the case that positive utility numbers are used for "goods" and negative utility numbers are used for "bads." Disutility is the sadness or lack of satisfaction a consumer gets from consuming an item. Something can also be both a "bad" and a "good" for different consumers. For instance, one person may love sourdough bread but another may be allergic to it. For the person with the allergy, the bread is a "bad," but for the person who loves it, the bread is a "good."
Cardinal versus Ordinal Properties of Utility
In contrast, the second part of the statement, that utility does not have cardinal properties, means utilities cannot be added or subtracted. In other words, a utility of 5 and a utility of 3 do not combine to give a utility of 8 (or anything else, for that matter). A related property is that a utility of 8 is twice as good as a utility of 4, and so on. In order to determine cardinal properties it is necessary to be able to use arithmetic, there needs to be numerical value given to the utility of options that are being compared.
Utility numbers are only considered in a sense of being bigger than or smaller than another utility number, and additional math is not done with them. Adding 5 to all of the utility numbers, multiplying them all by 3, and so on would not change what they represented in terms of preferences. If someone buying a houseplant rates a fern as a utility of 9 and a cactus with a utility of 4, adding these numbers together will not impact the fact that the fern has a higher utility and is thus preferred.