"The AWK programming language" has already been suggested and is great.

But awk is actually pretty simple. Imagine everything having an implicit loop like this around it like so:

  # BEGIN{ ... } would go here
  for $0 in input.split('\n'):
      $1, $2, ... = line.split()
      # if TEST {ACTION}; pairs ...
  # END { ... } would go here
      

so the first TEST in GP's awk would be `$1 ~ /class|module/` and the corresponding ACTION to take `{print $2}`.

if TEST is omitted it's True, if ACTION is it's print the whole line.