Most Drupal sites that allow user login are plagued by account requests from abusers and bots whose only intent is to post content to the site for nefarious purposes. While Drupal supplies tools that are effective at keeping them out (Captcha, for example, and administration approval of new accounts), there's still some level of overhead for administrators.
Mother May I provides a simple to use extra hurdle to reduce the bother of spamming account requests. It's primarily useful to sites with a limited target audience (at least for authenticated accounts) where valid users have some side information about the group. The site administrator can define a site-specific "secret word." Anyone requesting an account must enter the secret word before even a temporary account is created.
It's highly recommended that sites not depend solely on Mother May I to vet account requests, but this is an easy way to block spam requests from abusers that manage to get past the existing filters.
The site administrator specifies a secret word. A user requesting a new account must enter the secret word before account creation can proceed.