https://www.gamespot.com/articles/ps4-pro-update-will-make-big-graphical-improvement/1100-6456684/

Along with Sony's upcoming version 5.5 firmware update for the PlayStation 4, PS4 Pro owners that aren't on 4K televisions will be getting a significant graphical enhancement: supersample anti-aliasing (SSAA). PS4 Pro consoles will soon be able to leverage its extra power for non-4K displays and force SSAA when it outputs a 1080p resolution. This will be an option users have to enable manually, and it has been put into action with mostly positive results.

SSAA is a form of anti-aliasing that forces a game to render a higher resolution then downscale it to fit the proper screen resolution. The result is a smoother picture with less "jaggies" since you're essentially getting a shrunken version of a finer image. According Eurogamer's Digital Foundry, the PS4 Pro will act as if its connected to a 4K display and work to downscale the improved image to 1080p. While there are a few games that offer benefits with 1080p displays, some games (such as Metal Gear Solid V: The Phantom Pain) would lock out 1080p users from accessing any PS4 Pro benefits; an example of the inconsistency of the enhancements from the more powerful console. This will not be the case anymore with the new anti-aliasing option.