The tyre tread is the part that is actually in contact with whichever surface you are riding on. Especially in mountain biking this is incredibly important as it will provide the control you need for different types of terrain. For example, mud needs a tread that is more open and knobs that can dig into the terrain, but also have enough contact surface to provide traction on rocks, roots and the hardpack dirt beneath the mud. On road and urban tyres, the tread can help give more grip over rough or wet surfaces, by adding flexibility to the tread and conforming better to the surface.