What is the difference between the idea of stare decisis and something being unconstitutional?

The doctrine which obligates the courts for following the historical cases while making a ruling on a similar case is called Stare Decisis. It ensures the same approach towards the cases with similar scenarios. It binds courts for following the precedents set by earlier decisions. In Latin, it means " To stand by that which is decided" while being unconstitutional means that governmental action is in violation of the authority and rights which are granted by the constitution.

can you explain it better

the secretary of state, appointed by the president with the advice and consent of the senate, is the president's chief foreign affairs adviser. the secretary carries out the president's foreign policies through the state department and the foreign service of the united states.