An Einstein ring is a phenomenon that occurs when a massive object, like a galaxy or cluster of galaxies, bends the light from a more distant object, such as another galaxy. This effect is a result of the gravitational lensing predicted by Albert Einstein's theory of general relativity. When the distant object is perfectly aligned with the massive object, the light forms a ring-like structure around it. These rings can provide valuable information about the distribution of dark matter and the mass of the lensing object. Astronomers use Einstein rings to study the universe's structure and to understand how gravity influences light.