I was going through the paper on GAN by Ian Goodfellow. Under the Related work section, there is an equation(given in the image below), I cannot decipher the equation. Can anyone help me understand the meaning of the equation? Also, any guide to understanding mathematical notation for reading research paper are highly appreciated.
[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/eY8o6.png