Happiness is the fulfillment of desire, thus perfect happiness is the perfect fullfillment of the highest desire, that is the desire for God, or more exactly: ”The Vision of the Divine Essence”. Since happiness of this kind is eschatological the interesting thing is what Thomas calls ”imperfect happiness”, and especially how this relates to ”perfect happiness”.
In modern Christian thinking, at least the kind of thinking I was brought up on, the ”ethical universe” is essentially bi-polar. You either do what is good, or what is bad. No, that is not exactly right, a whole lot of things is ”neutral” in this kind of thinking, actions, thoughts etc that are not considered morally relevant, but part of secular life. Like economics. But I digress.
The point here is that Thomas’ moral universe is not bi-polar. All happiness is related to perfect happiness by participation. This of course follows from classical Christian ontology. What this means is that there is no such things as ”bad” happiness: If you are happy it is a good thing, and it means that you enjoy something that is related to the Creator God. What is important, of course, is if you are aware of this connection or not. This is the function of the talk of perfect happiness. Any happiness we experience here is by necessity imperfect, but unless we are aware of that we risk turning the quest for imperfect happiness to the goal of life, instead of looking for perfect happiness, that is, God.