Is that part of the frame that joins the saddle knot at the head tube. It is made of thinner thicknesses than the other tubes (at least in the center) because it undergoes less stress than other parts of the frame.
It is a key element for bike behavior and driving stability. To dampen the stresses and spread them throughout the structure, solutions are used that make this section a single piece with seat tubes and seat stay. So the chassis vibrations are distributed without disturbing the cyclist excessively.