The <<EOF syntax is known as a "here-doc", which you can read about in Quote and Quote like Operators.

I use it here to assign a multi-line string to a scalar variable. This was a convenient way for me to create a completely self-contained example which you could just copy-n-paste and run, without having to create 2 extra, external data files. Of course, in your application, you would read your 2 input files into your script.