Gravitational lensing is an effect on light from a background source that arises as a result of the curvature of spacetime, the three dimensions of space and time united into a single entity, caused by mass.
The effect is most observable when light from a bright background source, like a star, a quasar, or an entire galaxy, passes a very massive object like another galaxy or a cluster of galaxies, described as a lensing object or just a gravitational lens.
