That means all of its online services, including the game's eight-player multiplayer will not be available after that date.
You can still play the offline features of the game, of course. No troubles there.
It's a shame as our reviewer said that Retribution had "groundbreaking multiplayer, slick visuals, and an action-packed campaign."
But, well, that was nearly six years ago so presumably not too many people are playing it these days. And upholding online servers costs money. So it makes business sense that Sony is taking it down.
If you want to get any more online matches in on Retribution then you better get them out of the way quick.