The one tree God instructed Adam not to eat from is the tree of the knowledge of good and bad. God told Adam that if he ate from that tree he would positively die. This is the same tree that Satan, making a showing as a serpent, later convinced Eve that eating from this tree wouldn't result in death but would result in being like God by knowing good and bad.
Common misconception is that this tree's fruit was an apple. The Bible does not give this information.