Explanation:

Normal lenses such as the ones in a magnifying glass or a pair of spectacles work by bending light rays that pass through them in a process known as refraction, in order to focus the light somewhere (such as in your eye).
When astronomers refer to lensing, they are talking about an effect called gravitational lensing. Gravitational lensing works in an analogous way and is an effect of Einstein's theory of general relativity
– simply put, mass bends light.
The gravitational field of a massive object will extend far into space, and cause light rays passing close to that object (and thus through its gravitational field) to be bent and refocused somewhere else.
The more massive the object, the stronger its gravitational field and hence the greater the bending of light rays - just like using denser materials to make optical lenses results in a greater amount of refraction.
Application: This makes dim, faraway objects that otherwise would be undetectable, like an individual star, visible.
For e.g. NASA’s Hubble Space Telescope has discovered the farthest individual star ever seen — an enormous blue stellar body nicknamed Icarus located over halfway across the universe.