The phrase “forbidden fruit” refers to the story of Adam and Eve in the Garden of Eden. They were forbidden by God to eat the fruit of the tree of the knowledge of good and evil (Genesis 2:9, 3:2). The Bible says nothing about what type of fruit it was. Tradition has identified it as an apple, but it is impossible to know with certainty what kind of fruit it was. From the text of Genesis, every indication is of a literal tree with a literal fruit.
The key element in the passage is not the fruit itself, but the prohibition against eating it. God gave Adam and Eve only one prohibition in His instructions. Whether there was some spiritual property within the fruit is really irrelevant. The sin was in disobeying God’s command. By eating the fruit (an act of disobedience), Adam and Eve gained personal knowledge of evil. They already knew good, but now they had the contrasting experience of the evil of disobedience and the guilt and shame that came with it. Satan’s lie was that knowing good and evil would make them like gods (Genesis 3:5). In reality, they were already made in the image of God, and had the blessing of His good pleasure.
The lesson for us today is that when God prohibits something, it is for our own good. Disobeying Him, going our own way, or deciding for ourselves what is and is not beneficial to us will always lead to disaster. Our heavenly Father who created us knows what is best for us and when He prohibits something, we should listen to Him. When we choose to obey our own wills instead of His perfect and holy will, things never go well for us. Adam and Eve made that sad discovery after eating the forbidden fruit, and mankind has suffered the consequences of their decision ever since (Romans 5:12).