Generalized utility is the quantity which reality exists to maximize. Crucially, it is defined entirely within reality itself.
To understand this, consider that the universe is evolving and events are happening. They happen one way as opposed to any number of other ways. Taking reality as a whole because there is nothing outside of reality "real" enough to do anything to it, it must create itself. Given that it has total freedom of what to create itself as - why does it choose one form as opposed to any other? The answer is that one state, versus another, carries more self defined "utility" for reality. The most general formulation of this utility "drive" across the whole universe is called "Generalized Utility Function".
If a human being was asked about his motivation for doing anything- the "why?" behind his actions, he would point to a desire of some form. The global analogue of this desire, what the universal Desire hopes to achieve is "generalized utility".