Question

I’m trying to understand how the System class works.

When I go to the API, here, I see a field summary and method summary. I understand the method summary to be a list of all the methods. But what is the Field Summary?

And what are err, in, out, referring to?

Answer 1

As Nambari answered the first part of your question, here is the last part:

From the docs you linked from:

Field Summary
static PrintStream  err 
          The "standard" error output stream.
static InputStream  in 
          The "standard" input stream.
static PrintStream  out 
      The "standard" output stream.

err is where errors go, this can be redirected to file, console or wherever.

in is the input stream, be it file, keyboard, stream etc

out is the output, most often console or GUI based.