The special page Special:PasswordReset is useful in two cases:
Video Help:Reset password
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.
Maps Help:Reset password
Forgotten username
Similarly: If you have forgotten your username 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 and enter your email address, and the system will send an email containing, among other things, a reminder of your user name.
Source of the article : Wikipedia