Previously, by Justice Department custom, the president was immune from prosecution while in office. That was just an internal stance and didn’t make him immune from prosecution indefinitely, just that you can’t arrest the president in the Oval Office. It could be seen as a pragmatic stance that we shouldn’t tie the president up in court battles while he’s supposed to be doing the job of presidenting or that he’d just obviously order his Justice Department not to arrest him so don’t try to do irrelevant things.
Now he’s immune forever, even after he’s no longer in control of the people who would be prosecuting him and he has no critical role to play in society. And it’s officially declared by the Supreme Court, not just custom.