There are a couple of things circled, so I'll cover them all:
The largest diameter circle that meets with the top of the fork tube is called the fork cap. It's threaded into the top of the fork tube and allows access to the fork oil and other internals, as well as keeping them sealed away from the environment.
Inside the fork cap is the red hex adjuster, which is a preload adjuster. Screwing it in and out will change the amount of preload that is applied to the spring inside the fork. This will essentially make the fork feel softer or harder, change the ride height of the bike for a given weight, and slightly change the travel of the fork's stroke.
Inside the preload adjuster is a small screw head. That is likely your rebound adjuster. It adjusts how slow or fast the forks will extend after they have been compressed and the load has been removed. Too strong and the forks won't quickly return to their starting position ready for the next bump. Too soft and the bike can "pogo" and bounce up and down.
A common addon for sportbikes is an inconveniently named "preload adjuster". They are cheap metal pieces that fit over the nut and allow a rider to adjust preload with their bare hands. It's also possible this is what you're seeing.