"And the LORD God commanded the man, 'You are free to eat from any tree in the garden; but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die (Genesis 2:16-17 NIV).'"
Whenever we come to those sections of the Bible where all is not told we get tempted to fill-up the gaps. While it is indisputable that God gives revelations and deeper insights into His Word, some sections of the Bible sometimes attract 'wild' speculation and subjective claims of epiphany, that is, a sudden, intuitive perception of or insight into the reality or essential meaning of something. The result is confusion and doctrinal aberrations.
So, for example, what was this knowledge that God didn't want Adam and Eve to have a result of which He forbade them to eat from the tree in the middle of the garden of Eden? The answer is not in black and white. This makes it difficult to resist the temptation to speculate some hypothesis. And when we stray into this area it is not plausible to claim that we are giving a precise answer.
God had accomplished the work of creation and He saw that 'it was GOOD.' The first couple that He had just created in His own image must have appreciated the beauty of creation in the same way God did—'it was good.' In effect therefore, Adam and Eve 'knew' only 'The Good': the freshness and novelty of creation; the provision and fellowship; the innocence and security; the health and life; love and trust. What they didn't know was 'evil'. God had told them that the day they eat the fruit in the middle of the garden they will die. There had never been any death (they hadn't known death experientially, but they might have known it conceptually—otherwise why would God warn them with a consequence they didn't have an idea what it meant?).
If we stray a bit into semantics and idioms, we may realise that sometimes the word 'know' is used to refer to having an experience of something—practically as opposed to conceptually. So in essence, what God was telling Adam and Eve was that they will 'experience' (read: know) both good and evil. Again it is difficult to imagine what they perceived of the 'evil'. There hadn't been any that far. Nevertheless, they must have understood that 'evil' was something bad and should be avoided.
From the foregoing my final submission is that 'knowing good and evil' meant that they would come to 'appreciate' the 'true value' of good after they have lost it. As they say: 'You may not know the value of what you have until you lose it.'