"To improve the framerate" means to make a video game or animation run smoother and appear more fluid by increasing the number of images, or frames, displayed per second. Full definition
Dark Souls: Remastered includes the Artorias of the Abyss downloadable content, as well as improved framerate and resolution, Nintendo said in a release. (polygon.com)
Dark Souls is coming to #NintendoSwitch with improved framerate and resolution, as well as the Atorias of the Abyss DLC! (miketendo64.com)
[Update] According to the official Meta Gear Twitter account, the PS4 Pro patch increases the game resolution on a 4k screen while also improving the framerate. (wccftech.com)