The special page Special:PasswordReset is useful in two cases:

Forgotten password

If you have forgotten your password and you previously entered an email address when signing up for the account or in your Preferences, and you still have access to that email account, then this special page can help you recover access to your account.

Go to Special:PasswordReset. You can enter either your username or your email. If you are certain of your email, but not your username, only enter your email. The system will send a temporary password to your saved email address that will allow you to retrieve your account. You can change the password after you log in.

Note that you may have a preference enabled requiring you to enter both your username and email to receive a password reset.

Forgotten username

If you have forgotten your username you might be able to get the system to remind you.

If you:

...have previously entered an email address when signing up for the account or in your Preferences
...still have access to that email account
...have not enabled the preference requiring you to enter both your username and email to receive a password reset

then this special page can help you recover access to your account: Go to Special:PasswordReset and enter your email address, and the system will send an email containing, among other things[a], a reminder of your user name.

Technical restrictions

Notes

  1. ^ To be clear: this is exactly the same procedure as for resetting your password. Thus, this section ("Forgotten username") utilizes the fact that this email will not only contain a temporary password, it will also remind you of your username. If all you wanted was a reminder of your username, there you have it. You can keep using your old password.