configuring middleware documentation?

I will start by saying that I'm not actually familiar with Express, but I watched the video and I stayed at a Holiday Inn Express last night, so I think I can explain it.
I believe the by: is just a property name the instructor decided to use because it makes sense when you're reading it. This is much the same as any time you're writing a function and you are choosing the variable or parameter name to use in the function. In the actual function, he opts to use the name config for the parameter that is passed in from by. It looks like any time you want to pass a value like this, you configure your function with an outer function that takes a parameter, and you pass in the value via an object, and use that value in your function. I am assuming this information is correct. You could test it by putting in a different property name (like 'times') and confirm if you still get the correct results.