Glass, metal or plastic ruled with very close parallel lines. When light interacts with a diffraction grating, it is dispersed into a spectrum, similar to a prism.
Diffraction grating. (Source: https://prolabscientific.com/Diffraction-Gratings-6-X-12-Film-Sheets.html).
View of incandescent lights through a diffraction grating. Image courtesy of Robin Kingsburgh