Zip Mode, as the name implies, “will create a faster paced experience” by speeding up animations and shortening some delays. The patch also fixes a problem with shadow rendering causing slowdowns, adds support for SLI and Crossfire, enables triple buffering to fix Vsync issues, and does all sort of other sexy-sounding things, like “Removed MSAA options from default presets and adjusted autodetect to target 40FPS instead of 30FPS.” That sounds important, right? (Seriously, it probably is.) Slow communications during online multiplayer that resulted in delays between actions, turns, and the end-game screen have also been taken care of.
The patch should install automatically, but you may need to restart Steam to make it happen. You'll also have to take a couple of extra steps if you're using any mods: Launch XCOM 2, then exit, then launch it again, to ensure it has “internalized the patch to work with mods.” Most mods should play nicely with the patch as-is, but Firaxis warned that some of them may need to be updated.
Firaxis also recommended that players hit up the “Auto Detect Graphics Settings” option, because the patch won't automatically make any changes to the graphics settings.