What does Genesis 3:5 mean?

Here the serpent continues his deception of the first woman. His goal is to convince her to disobey God by eating from the Tree of the Knowledge of Good and Evil. In the previous verse, he flatly called God a liar for His warning that Adam and Eve would die if they ate the fruit from that tree. Now Satan reveals what he wants Eve to believe: that God's true motive for His rule is selfishness. According to the Devil, God just wants to scare them away so they don't become like Him. God is competitive and jealous. He can't be trusted to give commands for their good.

In fact, the serpent says, eating that fruit will open their eyes. They'll finally see the world as it really is, knowing all things: "good and evil," just like God. As in the previous verse, this deception contains a partial truth. Looking ahead a few verses, we see that mankind's eyes are opened. They do come to know good and evil. But that knowledge brings them neither God's power, nor His wisdom, nor His ability to love. Knowledge without corresponding maturity brings perversion. Humanity is not equipped for this knowledge, and so it brings them shame, fear, and pain. They come to know good by abandoning it. They gain the knowledge of evil by committing it for the first time in human history.

The power in the serpent's temptation was his attack on God's character and motivations: Don't obey God because He is neither good nor loving nor trustworthy. The Devil says God wants to rob us of experiencing true power, from gaining full understanding. This assumes that mankind is in a position to judge the character of God. That lie continues to drive humans toward sin and away from the good God who loves us.