THIS IS NOT A MODULE. YOU CAN'T ENABLE IT. FOLLOW THE INSTRUCTIONS.
Make a backup of your database before running this. It won't break your database, but we can't improve this unless we have broken databases that it doesn't quite do the job for.
There are times in Drupal 7 when the registry gets hopelessly hosed and you need to rebuild the registry (a list of PHP classes and the files they go with). Sometimes, though, you can't do this regular cache-clear activity because some class is required when the system is trying to bootstrap.
The drush version now also fixes Drupal 6 sites where modules have been moved around or removed and the system table is broken.
When would you need Registry Rebuild?
You might get something like:
PHP Fatal error: Class 'EntityAPIControllerExportable' not found in ...sites/all/modules/rules/includes/rules.core.inc on line 11
If this happens when you're trying to run update.php, and happens when you're trying to clear your cache, well, you have some trouble. That's what Registry Rebuild is for.
It also may happen that you've moved some module that Drupal requires to bootstrap, and you get a horrible error. Registry Rebuild will also rebuild the system table to get the modules in the right place so you can bootstrap.
When would you *not* need Registry Rebuild?