That is called the static strip, it is a material that prevents the buildup of static electricity as you ride.
"Static electricity build-up is an old concern that has returned to haunt modern tires. An inadequately grounded vehicle is an issue during refueling and when occupants slide out of the car’s interior. Modern tire compounds have become less conductive as manufacturers have greatly reduced the amount of carbon black in rubber compounds to cut weight and rolling resistance.
The solution is using what tire engineers call a grounding strip or an antenna tread designed into the tire. At the centerline of the tire, there’s a thin continuous strip of rubber with a compound that’s engineered to provide an efficient conductance path between the tire and the road surface. (The antenna tread design is quite evident in modern motorcycle tires.)"