SSAA – Supersampling
What this does is render frames at a higher resolution than your moniter can even display, and then samples it down. The opposite of this is like when you have a game that claims to be “4k” but actually just upsamples 1080p.
Starting with a higher resolution then scrunching it down to fit your monitor can result in better picture quality, but it comes at a high performance price. Once again, this is one of those settings where unless you’re running an absolute juggernaut of a gaming PC rig, you’re probably just fine to leave it off because the increase in overall performance will more than make up for anything you’ll be missing out on visually.