Waves reflecting from two surfaces can interfere constructively and destructively. In this case it is light waves that are being reflected at glass/air and air/glass interfaces. The interference produces a concentric ring pattern of rainbow colours in white light, or dark and light rings in monochromatic light.
The label reads "Συμβολή φωτός - Δακτύλιοι Νεύτωνος" which means "Interefernce of light - Newton's rings".