A magnifying glass is simply a glass lens that gathers and magnifies light. As illustrated below, a giant glass lens in space could collect the light from a distant galaxy and focus those light rays toward Earth. Astronomers would see that the galaxy is bigger and brighter than it would be without the lens.
Fortunately, nature provides its own intergalactic magnifying glass in large clusters of galaxies. The combined gravity of such galaxy clusters (below, Pandora's Cluster is shown) bends light in the same way that a glass lens does. Astronomers can use these galaxy clusters as "gravitational lenses" to see fainter, smaller, and more distant galaxies than otherwise possible.
Graphics: Ann Feild (STScI)