What is the gravitational lens effect?
The Gravitational Lens Effect is a massive object appearing to bend light.
As you may know all objects have gravity and all objects' gravity bends space around it. You might most commonly see this in a 2D plane being pulled down by a sun or planet.
The next thing to note is that light has no mass, but it does have momentum. Light will follow the curvature of space-time which is bent by all objects.
Now there is two laws interacting and you have super massive objects, like black holes, distorting huge amounts of space-time and light which is following the curvature of space-time. This interaction produces two images which have bent around the object.
Fortunately, though, scientists and mathematicians have developed formulas and technologies that allow them to reconstruct the image and accurately position the object.
(ALMA satellite dish observing SDP.81 after it has been bent by the supermassive black hole.)