17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
The one exception was not to eat from the tree of the knowledge of good and evil. There was probably nothing special about the fruit of this that would bring death other than that eating from it would be disobeying God. God was not being cruel but rather was giving man a test, and he was even making it as simple as possible. It was one tree among many and the command was extremely simple. The reason for the test was God did not create robots but wanted man to obey Him of his own free will.
The implications of this verse when seen in the light of chapter 3 are that man would start to die the day he eats of that tree. God had made Adam so well that the process of dieing took more than 900 years.
Note it was called “the tree of the knowledge of good and evil” and “the tree of the knowledge” as some have claimed it’s an important difference.