Sounds like common sense to me.
Don't bother with the opinion, it's really rather dull. The heart of the matter is this: California acknowledges its prisons violate the ban on cruel and unusual punishment. A three judge panel, after many years of failed attempts by the state of California to alleviate overcrowding, ordered a reduction in the percentage over capacity of prisoners in the CA system to 137.5% over capacity. California proposes to build more prisons and transfer prisoners out of state as a remedy, and says that because it has a workable plan the courts have no authority to order a reduction in capacity. Normally, under federal law, California would be right. The courts would be obligated to give them the time to see their plan through. But the panel ordered a reduction anyway because California's plan was identical to California's previous plans which hadn't worked, and because California, nearly bankrupt, clearly doesn't have the resources to follow through on that promise.
Kennedy upheld the reduction to 137.5%. It could be achieved entirely by building new prisons or transferring out prisoners, but if the state of California can't get the percentage low enough by so doing it must release prisoners.
Scalia and Alito (I think) do not dispute that the 8th amendment has been violated, and do not dispute that California is obligated to remedy overcrowding, but would allow California to continue with its proposed plan, even though it's guaranteed to fail, and even though it's as plain as day that California intends to drag its feet on prison overcrowding...well, forever if the courts will let it.