As Trump already knew, the president can't be prosecuted for carrying out acts that are within the executive branch powers granted to him in the Constitution. So now everyone knows this, and the special counsel's case will be dead on arrival at the lower court. Trump did nothing wrong and the special counsel was enlisted to do nothing more than hurt the former president's current campaign.
SCOTUS, with its conservative majority powered by three Trump nominees, has now guaranteed that no president — including Trump in a potential second term — will have any legal checks against potential criminal behavior. In effect, SCOTUS granted immunity beyond even what Trump's lawyers argued he had. This could push the US toward dictatorship.