Supersampling allows games that support enhanced textures, anti-aliasing, and resolutions above 1080p to scale to 1080p screens allowing for improved image quality, fewer jagged edges, and a sharper overall experience.
Although most PS4 Pro enhanced games let you choose between better graphics or a higher frame rate or run with supersampling, this ensures that those that don't now will. It's an interesting addition as the Xbox One X has had it since launch, making it obvious that Sony is looking to close the gap from a features standpoint.
Nonetheless, Sony does note that the outcome of this will differ from game to game. In some cases, the image quality may not be all that better or the frame rate may be impacted.
While this is a part of the PS4 system software 5.50 beta, it will be interesting to see if Sony makes it available to a wider audience when 5.50 is ready for mass consumption, or perhaps hold it back for more testing.
This isn't the first time Sony has brought core, system-level features to the PS4 Pro with an update. Last year saw Boost Mode which promised to increase the frame rate in games that weren't optimised for the PS4 Pro albeit it saw mixed results. How much of an impact supersampling has will be a point of contention in the weeks to come. Till then, here's a list of games from popular gaming forum ResetEra that support supersampling.