Just what it means is unclear to me. Who can guess the intent of its ancient author, or authors? We read into it what we want; interpretation is a matter of convenience, or doctrine, one or the other.
With that self-serving characterization in mind, I present my less than definitive, or for that matter less than definite, interpretation. That's to say, what I think about it. I think that it's language is clear enough and don't believe much in the way of interpretation is required, unless one is dismayed by the plain meaning of the words. I think one should be dismayed.
What the supposed first humans did that resulted in their banishment from the Garden and, according to Catholic doctrine in any case, tainted the human race forever after (in saecula saeculorum), was disobey a command of God. The wrong, as far as I can tell, was to disobey. That in itself merited punishment. Whether it was good or right or beneficial to eat of the Tree of Knowledge is not an issue--it isn't considered, even. Regardless of the reasons for the command, even if it was unreasonable, failure to obey a command of God is an evil in and of itself.
Plato in his Euthypro dealt with the question whether acting as commanded by God is good because God commands it, or whether God commands it because it is good. It can be argued that this story is an expression of the view that whatever God commands is good, not because it is good but because God commands it. Knowledge, after all, wouldn't seem to be necessarily evil. What is wrong with knowing?
Perhaps it can be argued that knowing, or knowing some things, causes us harm. In that case it may be maintained that God was trying to prevent us from being harmed. In fact, God warns Adam not to eat the fruit of the Tree because if he does, he'll die. It's not clear to me why eating of the Tree of Knowledge causes death, and this isn't explained in the text. Perhaps the knowledge gained by eating the forbidden apple in fact led to our undoing, if not death. We came to think, and came to think of ourselves as separate and apart from the rest of the world, rather than living with nature in blissful harmony in the Garden.
If God was trying to prevent us from harming ourselves, though, it would seem perverse of him to cause us significant harm because we harmed ourselves. Expulsion from the Garden exposed us to a life of pain and misery; doomed us to it, in fact, rendered us subject to eternal punishment if not appropriately saved. It seems a remarkably high price to pay assuming a God devoted to our well being.
It's said that the knowledge gained was the knowledge of good and evil. This is puzzling, to me at least. It seems that in the blessed state of inhabitants of the Garden of Eden, the parents of humanity possessed no such knowledge. Not knowing good and evil, they didn't even know what those words meant. They lived not only without knowledge but without judgment or thought, which would be necessary to distinguish good from evil. They were, it seems, much like the rest of the animals. Thoughtless. The knowledge of good and evil renders us like God.
It's difficult not to conclude that the Sin for which we were punished was that of having the capacity to think and to make judgments and decisions. That view of sin is one which an autocrat or elite might find useful. If thinking is a sin, thinking is something to be avoided lest we suffer eternal damnation. This would seem to suit those in positions of power quite well. Those who don't think are easily manipulated.
I'm uncertain of the origin of the phrase "ignorance is bliss" but this would appear to be what we can infer is the moral of the tale if we accept the tale as written, as told. I'm certain, however, that this inference has been avoided for quite some time and alternate explanations offered. I'm always suspicious when interpretation is made so lavishly of easily comprehended language, however.