A diffraction grating is a device that is used to diffuse light and obtain spectrum, diffraction here means bending. The light goes straight, but it has a wave nature and spreads when it passes through the a narrow hole (slit) because the wave has a nature of spreading in all directions. Diffraction grating means a glass plate with so many lines drawn on it. Specifically, if the glass plate is burned, it will become sooty black. Drawing lines at regular intervals here will clear the soot, and create gaps for light to pass through. This is how diffraction grating is made.