Assuming the software is properly configured for email this dialog will send an email to a user if he/she forgets his/her password. Users may access this form from the 'I've forgotten my password' link on the login dialog.
The userid must have an email associated with it in order for this feature to work (otherwise there would be no one to whom to send the reminder). For this reason, if you wish to use the feature, you should probably make the email address required under the user settings dialog.
Disabling this feature
You may disable this feature under the user settings dialog. Uncheck the 'User Registration | Allow emailed password reminders' checkbox. Note that this feature is only enabled if the email settings are properly set, so another way to disable this feature is to disabled the email feature.