The cardinal utility theory treats utility as something that can be measured in absolute terms. We can measure a person and say the person is 1.7 meters tall. So too, these theorists say, we can measure the utility a person gets from a good or service and say they get 1.7 utils, for example, from it.
This is in contrast to ordinal utility theory which simply says that a person gets more utility from good A than good B.
Cardinal utility is helpful for teaching about marginal utility and things like that. However, it seems to me that it is quite impossible to measure utility in a cardinal way. I can only talk about how much satsifaction I get from buying a book by comparing it to what I get from watching a movie. I cannot put a number on the book without reference to other things.