[development] converting a module from Drupal 5 to Drupal 6

[Sheryl (Permutations Software)]

I'm converting a module from Drupal 5 to Drupal 6, and I'm having a very
hard time understanding the change in the use of 'op'. I can't figure out
how I need to change my code. There are these lines.

  if (!empty($form_values['op'])) {
    _phpfreechat_clear_cache();
    _phpfreechat_clear_cookies();
  }
  if (!_phpfreechat_check_install()) {
    form_set_error('op', _phpfreechat_not_found());
  }
  if (!_phpfreechat_check_files()) {
    form_set_error('op', t('Unable to create data folders for
phpfreechat.'));
  }

I took this module over from someone else, so I don't fully understand the
use of 'op' here to begin with. Not understanding what it's doing makes it
very hard to rewrite. I've spent the last two hours on the Drupal site
trying to figure it out and I'm boggled. All I can tell is that 'op' was the
default name of buttons in Drupal 5, but I still can't figure out what's
being tested in these "if" statements or how I need to rewrite them for
Drupal 6. Any guidance you can give me would be greatly appreciated!

- Sheryl