When God created human He placed them in the garden of Eden where there was all kind of animals and plants. In the centre of the garden, God had planted the tree of wisdom and knowledge which is also known as the forbidden tree.
God instructed Adam and Eve not to eat the fruits from the forbidden tree because He knew that it would destroy their innocence and purity. The fruit from the tree of wisdom would introduce shame and guilt, and bring death into their lives. God told Adam if he ate from the tree he would surely die.
Because of this, Adam knew that he was supposed to obey God, but he had no real experiential understanding about death because he had not seen it. After the devil deceived them to eat the fruit, they got aware of their nakedness, filled with shame, cursed with difficulties and kicked out of the garden.
Their disobedience caused a ripple effect of sin and death that stretched beyond themselves to all of humanity in the world across all the time.