It means, cynically, that the president may be prosecuted if the courts deem the action "unofficial" and not otherwise. Which is to say that the court has removed checks and balances for the case where the SCOTUS and Executive branch are held by the same party.

Cleary, clearly this was a partisan decision. They can't just say "We Have a King Now", so they dressed up just enough of a reasonable interpretation to be able to kill this particular prosecution, while allowing themselves wiggle room to prosecute the kings they don't like. They aren't really trying to uncork executive abuse, they really hope it doesn't happen. But they want Trump not to be prosecuted, and put their fingers on the scale with what they hope is just enough pressure. We'll see.