What exactly is it that is being magnified 50 times in this gravitational lensing observation?
Reading through the full article, I see that the authors write


  we estimated the lensing amplification to be $\mu\sim52$.


This refers to the magnification factor. Essentially, it describes the solid angle of the image, as related to the solid angle of the source1:
$$\mu\equiv\frac{\theta}{\beta}\frac{d\theta}{d\beta}$$
Here's a diagram of the magnitudes of the angles:



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