FSD is Tesla's "Full Self Driving" - a product marketed as fully autonomous driving (like the full name suggests), that people paid for and then were told to go kick rocks by an idiotic CEO when it never materialised. Currently it comprises of things like lane assist and cruise control, but is anything but fully autonomous as the driver has to stay alert and "ready to take over" at any point. It's very poorly implemented, using cameras only (someone who think's he knows better than all the other car makes told his engineers to remove the LIDAR used on older models). Tesla got outed faking a video of it in operation several years ago. It also has a habit of running into the back of stationary emergency vehicles, motorcycles, cyclists, trees and lane dividers.
The level 5 refers to SAE standards for autonomous vehicles, which is the nirvana of self driving e.g. you can kip in the back and get to your destination, alive, rather than the sub L3 it's currently at with driver assist features.